The current traversal logic is spread between ReactFiberWorkLoop and
ReactFiberCommitWork, and it's a bit awkward, especially when
refactoring. Idk the ideal module structure, so for now I'd rather keep
it all in one file.

We suspect that using the JS stack to traverse through the tree in the
commit phase is slower than traversing iteratively.

I've kept the recursive implementation behind a flag, both so we have
the option to run an experiment comparing the two, and so we can revert
it easily later if needed.