fix: allow Data to be inferred from Handler function

[csvn]

This PR adds support for single-function GET handler data to be inferred when using define.page<typeof handler>().

export const handler = define.handlers((ctx) => {
  return page({ foo: "foo" });
});

export default define.page<typeof handler>((ctx) => {
  ctx.data; // Correctly resolves as { foo: string }
});

This PR also adds a test that checks that the types inferred are correct by type checking.

[marvinhagemeister]

Nice, thanks!