flushSync should change priority of commit phase updates to sync, even though it can't flush them immediately

[acdlite]

flushSync early returns if it detects it's in the commit (or render) phase, since it can't synchronously flush when React is already working.

However, it should still change the priority of updates scheduled by the callback to be synchronous.

This mostly already works because the commit phase itself has a sync event priority context, but the passive effect phase doesn't.

Need more tests for this.