Idle updates should not be blocked by hidden work (#16871)

* Idle updates should not be blocked by hidden work

Use the special `Idle` expiration time for updates that are triggered at
Scheduler's `IdlePriority`, instead of `Never`.

The key difference between Idle and Never¹ is that Never work can be
committed in an inconsistent state without tearing the UI. The main
example is offscreen content, like a hidden subtree.

¹ "Never" isn't the best name. I originally called it that because it
"never" expires, but neither does Idle. Since it's mostly used for
offscreen subtrees, we could call it "Offscreen." However, it's also
used for dehydrated Suspense boundaries, which are inconsistent in the
sense that they haven't finished yet, but aren't visibly inconsistent
because the server rendered HTML matches what the hydrated tree would
look like.

* Reset as early as possible using local variable

* Updates in a hidden effect should be Idle

I had made them Never to avoid an extra render when a hidden effect
updates the hidden component -- if they are Idle, we have to render once
at Idle, which bails out on the hidden subtree, then again at Never to
actually process the update -- but the problem of needing an extra
render pass to bail out hidden updates already exists and we should fix
that properly instead of adding yet another special case.