feat: add support for route groups

[marvinhagemeister]

This PR adds support for route groups which allows you to use different layouts for routes in the same URL segment.

/routes
  /(marketing)
    /_layout.tsx   <-- only applies to about.tsx and career.tsx
    /about.tsx
    /career.tsx
  /(info)
    /_layout.tsx   <-- only applies to index.tsx and contact.tsx
    /index.tsx
    /contact.tsx

To be able to do this we need to keep track of the original route paths. Previously, matching _layout and _middleware was done via their URLPattern, but this doesn't work with groups anymore. With groups multiple _layout files can have the same URLPattern and it becomes ambiguous as to which file to pick.

To solve this we need to match on the original file path semantics instead of their resulting URLPattern. To do that the PR adds an additional property baseRoute to each route which is the normalised file path. When a route matches we can retrieve that (like /foo/(bar)/(bob)/about) and easily match the needed layout files.

The PR is a bit longer than I'd hoped. Most of the changes relate to changing the internal route signature from:

interface Route {
  [K in "GET" | "PATCH" ...]: ...
}

to

interface Route {
  baseRoute: string;
  methods: {
    [K in "GET" | "PATCH" ...]: ...
  }
}

Instead of making baseRoute just a string, I made it a nominal type. Noticed that I ran into a couple of errors during development where some values started with a /, others didn't etc. With the nominal type we can force it to have been parsed first and ensure it always has the expected format.

Fixes #1150 .

[marvinhagemeister]

oh no, looks like I broke some tests 🙈

[marvinhagemeister]

Alright, ready to review now 🎉

[bartlomieju]

LGTM, no comments. I love the approach of using a nominal type 👍