[App Shells] staged shell rendering during build (#94442)
The final PR in the initial App Shell series. Here, we turn the
build-time static prerender into a proper staged render, and recover a
shell from it. The only difference between them is static link data,
i.e. root params and static params. Those are delayed until the static
stage analogously to the previous PRs.
We collect the chunks emitted in the shell stages and emit them as a
special `/_shell` segment marked with an `isPartial` byte.
