Add type-safe context state

[EthanThatOneKid]

Changes

Adds state to class Router.

const statefulRouter = createRouter(
  (r) =>
    r
      .get(
        "/*",
        async (ctx) => {
          ctx.state.user = { name: "Alice" };
          return await ctx.next();
        },
      )
      .get<"name">(
        "/:name",
        (ctx) => {
          if (ctx.state.user === null) {
            return new Response("Unauthorized", { status: 401 });
          }

          if (ctx.state.user.name !== ctx.params.name) {
            return new Response("Forbidden", { status: 403 });
          }

          return new Response(`Hello, ${ctx.params.name}!`);
        },
      ),
  (): { user: User | null } => ({ user: null }),
);

interface User {
  name: string;
}

Implications of updated Router.fetch API

• Possible disadvantage: internal variables exposed to library user. The fetch method is intended to be called with its first argument, request, so url, state, i are intended for internal use only, despite allowing developers to pass their own values.
• Advantage: The values of the arguments url, state, and i are only evaluated as necessary; the values are preserved in subsequent recursive calls.