Unify use and renderDidSuspendDelayIfPossible implementations

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When unwrapping a promise with use, we sometimes suspend the work loop from rendering anything else until the data has resolved. This is different from how Suspense works in the old throw-a-promise world, where rather than suspend rendering midway through the render phase, we prepare a fallback and block the commit at the end, if necessary; however, the logic for determining whether it's OK to block is the same. The implementation is only incidentally different because it happens in two different parts of the code. This means for use, we end up doing the same checks twice, which is wasteful in terms of computation, but also introduces a risk that the logic will accidentally diverge.

This unifies the implementation by moving it into the SuspenseContext module. Most of the logic for deciding whether to suspend is already performed in the begin phase of SuspenseComponent, so it makes sense to store that information on the stack rather than recompute it on demand.

The way I've chosen to model this is to track whether the work loop is rendering inside the "shell" of the tree. The shell is defined as the part of the tree that's visible in the current UI. Once we enter a new Suspense boundary (or a hidden Offscreen boundary, which acts a Suspense boundary), we're no longer in the shell. This is already how Suspense behavior was modeled in terms of UX, so using this concept directly in the implementation turns out to result in less code than before.

For the most part, this is purely an internal refactor, though it does fix a bug in the use implementation related to nested Suspense boundaries. I wouldn't be surprised if it happens to fix other bugs that we haven't yet discovered, especially around Offscreen. I'll add more tests as I think of them.

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