dillon1000/react

Move sync task queue to its own module (#21109)

The sync task queue is React-specific and doesn't really have anything
to do with Scheduler. We'd keep using it even once `postTask` exists.

By separating that part out, `SchedulerWithReactIntegration` is now
just a module that re-exports the Scheduler API. So I unforked it.
When we switch to ES Modules, we can remove this re-exporting module.