refactor <AppRouter /> structure (#64594)
Continuation of https://github.com/vercel/next.js/pull/59606
Previous PR description, for posterity:
> The current composition of RSC and client layers is a little awkward.
In particular the AppRouter component is a client component but it is
rendered by RSC which makes the intial data load format for RSC
different than what you would get on a navigation. This change moves
around some implementations to lift AppRouter out of the RSC tree and
make it render as part of the Client entrypoint directly.
Additionally, this PR unlocks some future optimization work, such as
re-using the SSR stream for generating FlightData & ensuring preloads
are properly captured during `generateFlight`.
Fixes #53274
As as we'll no longer be serializing `initialCanonicalUrl` in favor of a
shortened variable
