Unify appShells flag with Partial Prefetching (#95415)
The experimental `appShells` flag was added while the feature was being
developed. It was not meant to be a public-facing flag.
Some of the behavior that was gated behind the `appShells` flag is
purely an optimization and can be landed without any gate.
There are also some behaviors that we don't want to turn on unless
you've opted into the new prefetching model via Partial Prefetching.
Arguably these too are internal optimizations, but because they can
change the number of requests that are made to the server (e.g. one
request for the shell, another for the page data), we will gate these
behind Partial Prefetching to minimize the impact on existing
applications.
The result is that until you opt into Partial Prefetching, you should
not see any meaningfully detrimental impact to prefetching costs,
regardless of how your app is structured.
