Add experimental.appShells feature flag (#93997)
Plumbing for upcoming App Shell prefetching support: sets up the config
field, the zod entry, and the build-time `process.env.__NEXT_APP_SHELLS`
define. No behavior change in this PR — nothing reads the flag yet.
Subsequent PRs in this sequence wire the server- and client-side work
behind it.
Enabling `experimental.appShells` requires several adjacent flags to
also be enabled: `cacheComponents`, `experimental.prefetchInlining`,
`experimental.varyParams`, `experimental.optimisticRouting`, and
`experimental.cachedNavigations`. All of these are on track to become
defaults soon, so there's no value in supporting App Shells against
arbitrary subsets — the validation enforces that App Shells is tested in
the configuration it will ship with. Each requirement drops out as the
corresponding flag becomes a default.
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