dillon1000/react

Remove Task priority (#11307)

* Remove Task priority

The concept of Task priority was originally added as a way to avoid
reentrancy. Sync priority is for work that flushes synchronously, and
Task is for work that flushes at the end of the event loop.

But it turns out that in most cases, it's simpler to model Task and
Sync as the same priority level. For example, it's never correct to
flush Sync work from the update queue without flushing Task. Doing so
can lead to infinite update loops.

And using a separate priority level is not necessary to avoid
reentrancy. We already track when work is being rendered, and exit
before entering the render cycle again. That alone is sufficient.

This commit removes Task priority from the codebase. Now we use the
same level for both truly synchronous work and work that is deferred
until the end of the event loop.

This also enables us to remove DOM-specific legacy cases from the 
reconciler and lift them to the renderer.

* Remove isUnbatched from FiberRoot

Simpler to render unbatched roots immediately.

* Use a cyclic linked list for root schedule

Avoids the need for a separate `isScheduled` field.