router: implement route interceptions (#47565)
This PR implements the route intercepting feature from [the layouts
RFC](https://nextjs.org/blog/layouts-rfc#intercepting-routes).
You can use route interception by adding these special markers,
`(..),(...) or (..)(..)` to your segment definition, i.e. by creating a
page with this path: `/foo/bar/(..)baz/page.ts`.
Changes in this PR:
- added a new kind of route handlers to support intercepting routes
- added a "referrer" concept that is derived from the client router
state on navigation, where we try to guess which URL you were on before
- added some units tests
- updated the renderer to allow for overrides when the path params don't
include the current dynamic param, so we rely on the passed router state
to create it
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fix NEXT-749 ([link](https://linear.app/vercel/issue/NEXT-749))
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Co-authored-by: Wyatt Johnson <accounts+github@wyattjoh.ca>
