dillon1000/react

[Scheduler] Call postTask directly (#19551)

This updates the experimental Scheduler postTask build to call postTask
directly, instead of managing our own custom queue and work loop.

We still use a deadline 5ms mechanism to implement `shouldYield`.

The main thing that postTask is currently missing is the continuation
feature — when yielding to the main thread, the yielding task is sent
to the back of the queue, instead of maintaining its position.

While this would be nice to have, even without it, postTask may be good
enough to replace our userspace implementation.

We'll run some tests to see.