Support property type overrides when override type is compatible with parent type

[jasongwartz]

pkl-typescript currently doesn't support generating child classes that override the type of a property from the parent class.

The README currently says:

TypeScript interfaces, when using extends to extend a parent type, do not support overriding the type of a property from the parent.

This is actually only sort-of true - TypeScript does allow overriding a property type when the new type is compatible with the parent type (eg. narrowing string to a string literal type).

If it's possible to detect this compatibility in Pkl reflection, we should allow this particular subclassing behaviour.

[jasongwartz]

Ref:

pkl-typescript/codegen/src/internal/ClassGen.pkl

Line 81 in ace342d

else if (superProp == null || isSameType(superProp.type, prop.type)) true

and

pkl-typescript/codegen/src/internal/ClassGen.pkl

Line 47 in ace342d

local function isSameType(typeA: reflect.Type, typeB: reflect.Type) =

[jasongwartz]

Partially resolved by: #38 (for string-type to string-literal type)