Improved TypeScript types & type tests, for better type inference

[tgriesser]

A scaled down version of #118, only updating the itty-router.d.ts file:

Summary of type changes:

• Router<Request, Methods> -> Router<{ methodExtension: Methods }>
• Request defaults to the current global Request, any extension can be made via declaration merging
  • This significantly improves the type behavior, and eliminates the need to specify the Request on individual route handlers

Additional features:

• Auto-inference of type params /some/:path/:id? -> req.params is { path: string, id?: string | undefined } without any additional work
• Auto-inference of the appropriate HTTP method on the req.method property
• Ability to supply the handler args Router<{ handlerArgs: [Env, ExecutionContext] }>, which means they'll be typed correctly both on .handle, and on all route handlers automatically
• Ability to enforce the return-type of the handler via Router<{ handlerReturn: Response }>
  • Used to catch cases where you are accidentally returning something unexpected, typically you'd want Response | void if you support passthrough handlers, but it defaults to any to match current behavior
• Nothing Cloudflare specific in the types, added test cases to ensure the proper behavior of the Request is as expected when the @cloudflare/workers-types are in the global scope
• Adds a ton of positive & negative test-assertions for the expected types

[kwhitley]

I'm in the middle of a massive overhaul, bringing extras into core, with a full rewrite in TS... I need to tease this issue/PR back apart to see how to proceed, but no matter what, I'd like to be able to merge something so you end up on the contributors list :)

I'll let you know if I need it dialed back so we can get your name on the board... stay tuned!

[kwhitley]

Just added this request for help https://github.com/kwhitley/itty-router/issues/122... but you may be a good candidate for saving the day! :)

[kwhitley]

I'm gonna close this for now, as v3 is out (with a complete TS overhaul), but feel free to reopen to discuss further, or hop on Discord (ideally) to continue the conversation there! :)