dillon1000/react

[Fiber] Avoid return value from commitBeforeMutationEffects (#31922)

This is behind an unusual flag (enableCreateEventHandleAPI) that doesn't
serve a special return value. I'll be collecting other flags from this
phase too.

We can just use the global flag and reset it before the next mutation
phase. Unlike focusedInstanceHandle this doesn't leak any memory in the
meantime.