dillon1000/react

Fix priority of clean-up function on deletion (#16277)

The clean-up function of a passive effect (`useEffect`) usually fires
in a post-commit task, after the browser has painted. However, there is
an exception when the component (or its parent) is deleted from the
tree. In that case, we fire the clean-up function during the
synchronous commit phase, the same phase we use for layout effects.

This is a concession to implementation complexity. Calling it in the
passive effect phase would require either traversing the children of the
deleted fiber again, or including unmount effects as part of the fiber
effect list.

Because the functions are called during the sync phase in this case,
the Scheduler priority is Immediate (the one used for layout) instead
of Normal. We may want to reconsider this trade off later.