Loose types of app routes return value (#55849)
### What
#51394 introduced a pretty strict type of return value of route type
that causing failure with `next build`.
There're few ways of writing a app route, it could contain few return
values based on the usage:
* return a `Response` or promise of it
* return `NextResponse` of promise of it, since it's extended from
`Response`, same type
* use `redirect()` or `notFound(), since it returns `never`, and the
below code is not reached, the handler itself could still return void.
e.g. using `redirect` in a `GET` route
We loosed the type so `redirect()` can be still allowed without
specifying the return value there.
Related typescript issue:
https://github.com/microsoft/TypeScript/issues/16608#issuecomment-309327984
### How
* Re-enable the bail on types / build error in the app-routes tests
* Separate the tests, move runtime erroring ones to
`test/e2e/app-dir/app-routes-errors`
* Add new case to app-routes tests of mixed return value
Closes #55623
Related #55604
