Typesafe middleware route matcher

[Enriquefft]

Goals

1. Be able to use as const within middleware config to reuse the matcher information.
2. Be able to use the asserts keyword.
3. Be able to combine the typedRoutes experimental feature with the middleware matcher.

Background

NextJs is TypeScript-first, which is what makes it so good, but this small feature can be an easily solvable source of errors when explicitlyf defining the route matcher.

Proposal

I'm aware there is a good reason for some typescript keywords to not be available withtin NextJs config, so as an alternative it could be possible to extend the typedRoutes experimental config so that it also applies in the route matcher.