Add README, and basic example (#22)

* Add README, and basic example

* Add note about current known limitations

README.md

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+> [!CAUTION]
+>
+> # THIS LIBRARY IS CURRENTLY PRE-RELEASE
+>
+> `pkl-typescript` is currently major version `v0`, and **breaking changes will happen** between versions.
+>
+> Please read the section [Roadmap](#roadmap) below to learn more.
+
+# Pkl Bindings for TypeScript
+
+This library exposes TypeScript language bindings for the Pkl configuration language.
+
+These language bindings are made up of:
+
+- an "evaluator", that can execute Pkl code and deserialise the result into JavaScript runtime objects
+- the `pkl-gen-typescript` CLI, that can analyse a Pkl schema and generate matching TypeScript definitions
+
+Together, this allows you to embed Pkl into your TypeScript application, complete with code generation for full type safety and ease of use.
+
+## Getting Started
+
+First, install `pkl-typescript` from NPM:
+
+```bash
+npm install @pkl-community/pkl-typescript
+```
+
+Then, generate a TypeScript file from your Pkl schema (eg. for a file called "config.pkl"):
+
+```bash
+npx pkl-gen-typescript config.pkl -o ./generated
+```
+
+Lastly, in your TypeScript code, you can import the generated types and loader code:
+
+```typescript
+import { type Config, loadFromPath } from "./generated/config.pkl.ts";
+
+const config: Config = await loadFromPath("config.pkl");
+```
+
+See more example usage in the [examples directory](./examples/).
+
+### Note on Schemas vs. Configs
+
+`pkl-gen-typescript` generates a TypeScript file based on Pkl's **type information** only, _not_ Pkl's runtime values. For example, a Pkl file with `x: String = "hello"` would produce the TypeScript type `x: string`.  
+Conversely, the evaluator (used by the `loadFromPath(string)` function) evaluates a Pkl module that **renders values**.
+
+You may choose to have your Pkl schemas and values defined in separate Pkl files (eg. `schema.pkl` and `config.pkl`, where `config.pkl` starts with `amends "schema.pkl"`). In such a case, you would pass `schema.pkl` to `pkl-gen-typescript`, but then evaluate `config.pkl` at runtime (ie. `await loadFromPath("config.pkl")`).
+
+## Roadmap
+
+This library is currently in pre-release: we believe it is usable and productive in its current state, but not feature-complete, and not yet API-stable.
+
+We will keep the major version at `v0` until we are ready to commit to stability in:
+
+- the evaluator API (as provided by the `@pkl-community/pkl-typescript` NPM package)
+- the TypeScript type definitions generated by `pkl-gen-typescript`
+
+Until then, minor and patch releases may contain breaking changes.
+
+> [!WARNING]  
+> **We strongly recommend** you regenerate your generated TypeScript code (with `pkl-gen-typescript`) **every time you upgrade** `@pkl-community/pkl-typescript`. If you don't, you may end up with unexpected runtime errors from type mismatches.
+
+### Known Current Limitations
+
+- **Inlined imports**: Imported Pkl types are inlined into the output TypeScript file. For example, if `foo.pkl` has an import like `import "bar.pkl"`, and you run `pkl-gen-typescript foo.pkl`, the resulting `foo.pkl.ts` file will include all types defined in `foo.pkl` _as well as_ all types defined in `bar.pkl`. This means that the resulting TypeScript generated files (in a multi-file codegen) will match the set of input root files, not the file structure of the source Pkl files. This behaviour may create unintended name conflicts; these can be resolved using the `@typescript.Name { value = "..." }` annotation. It may also cause duplication (eg. if the same shared Pkl library file is imported in two schemas); TypeScript's structural typing (where equivalent type shapes can be used interchangeably) should mean that any duplicate types can be safely used as each other.
+- **Regex deserialisation**: Pkl's `Regex` type will be decoded as a `pklTypescript.Regex` object, which contains a `.pattern` property. Pkl uses Java's regular expression syntax, which may not always be perfectly compatible with JavaScript's regular expression syntax. If you want to use your Pkl `Regex` as a JavaScript `RegExp`, and you are confident that the expression will behave the same way in JavaScript as in Pkl, you can instantiate a new `RegExp` using the `pklTypescript.Regex.pattern` property, eg. `const myConfigRegexp = new RegExp(myConfig.someRegex.pattern)`.
+- **IntSeq deserialisation**: Pkl's `IntSeq` type is intended to be used internally within a Pkl program to create a range loop. It is unlikely to be useful as a property type in JavaScript, and is therefore decoded into a custom `pklTypescript.IntSeq` type with signature `{ start: number; end: number: step: number }` - it is _not_ decoded into an array containing the ranged values. If you have a use-case to use `IntSeq` as an array of ranged values in a TypeScript program, please file a GitHub Issue.
+- **Duration and DataSize APIs**: Pkl has a rich API for many of its custom types, but two of note (that are not common in standard libraries of other languages) are `Duration` and `DataSize`, which include convenience APIs for eg. converting between units or summing values. These types are decoded into `pklTypescript.DataSize`/`pklTypescript.Duration` types (each of which have a `value` and `unit` property), and do not yet have the convenience APIs from Pkl.
+
+## Appendix
+
+### Type Mappings
+
+When code-generating TypeScript type definitions from Pkl schemas, each Pkl type is converted to an associated TypeScript type, as per the table below. While in pre-release, these mappings are subject to change!
+
+| Pkl type         | TypeScript type            |
+| ---------------- | -------------------------- |
+| Null             | `null`                     |
+| Boolean          | `boolean`                  |
+| String           | `string`                   |
+| Int              | `number`                   |
+| Int8             | `number`                   |
+| Int16            | `number`                   |
+| Int32            | `number`                   |
+| UInt             | `number`                   |
+| UInt8            | `number`                   |
+| UInt16           | `number`                   |
+| UInt32           | `number`                   |
+| Float            | `number`                   |
+| Number           | `number`                   |
+| List<T>          | `Array<T>`                 |
+| Listing<T>       | `Array<T>`                 |
+| Map<K, V>        | `Map<K, V>`                |
+| Mapping<K, V>    | `Map<K, V>`                |
+| Set<T>           | `Set<T>`                   |
+| Pair<A, B>       | `pklTypescript.Pair<A, B>` |
+| Dynamic          | `pklTypescript.Dynamic`    |
+| DataSize         | `pklTypescript.DataSize`   |
+| Duration         | `pklTypescript.Duration`   |
+| IntSeq           | `pklTypescript.IntSeq`     |
+| Class            | `interface`                |
+| TypeAlias        | `typealias`                |
+| Any              | `pklTypescript.Any`        |
+| Unions (A\|B\|C) | `A\|B\|C`                  |
+| Regex            | `pklTypescript.Regex`      |

examples/basic-intro/config.pkl

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+firstName: String = "Phillip"
+lastName: String = "Pklton"
+age: Int8 = 76

examples/basic-intro/generated/config.pkl.ts

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+// This file was generated by `pkl-typescript` from Pkl module `config`.
+// DO NOT EDIT.
+import * as pklTypescript from "@pkl-community/pkl-typescript"
+
+// Ref: Module root.
+export interface Config {
+  firstName: string
+
+  lastName: string
+
+  age: number
+}
+
+// LoadFromPath loads the pkl module at the given path and evaluates it into a Config
+export const loadFromPath = async (path: string): Promise<Config> => {
+  const evaluator = await pklTypescript.newEvaluator(pklTypescript.PreconfiguredOptions);
+  try {
+    const result = await load(evaluator, pklTypescript.FileSource(path));
+    return result
+  } finally {
+    evaluator.close()
+  }
+};
+
+export const load = (evaluator: pklTypescript.Evaluator, source: pklTypescript.ModuleSource): Promise<Config> =>
+  evaluator.evaluateModule(source) as Promise<Config>;

examples/basic-intro/index.ts

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+import { type Config, loadFromPath } from "./generated/config.pkl";
+
+const main = async () => {
+  const config: Config = await loadFromPath("config.pkl");
+  console.log(
+    `Hello, ${config.firstName} ${config.lastName}! I hear you are ${config.age} years old.`
+  );
+};
+
+main();