dillon1000/react

Rename ReactFiberScheduler -> ReactFiberWorkLoop and extract throwException from Unwind (#15725)

* Rename ReactFiberScheduler to ReactFiberWorkLoop

The scheduling part is mostly extracted out to the scheduler package.

What's remaining is mostly around the loop around each section of work.
I name it something with Work in it because it's very related to the
BeginWork, CompleteWork and UnwindWork sections.

* Extract throwException from UnwindWork

Our throwing works more like algebraic effects in that it's a separate
phase where we find a handler and we later unwind.