dillon1000/react

[Partial Hydration] Dispatching events should not work until hydration commits (#16532)

* Refactor a bit to use less property access

* Add test for invoking an event before mount

* Add Hydration effect tag

This is equivalent to a "Placement" effect in that it's a new insertion
to the tree but it doesn't need an actual mutation.

It is only used to determine if a subtree has actually mounted yet.

* Use the Hydration flag for Roots

Previous roots had a Placement flag on them as a hack for this case but
since we have a special flag for it now, we can just use that.

* Add Flare test