dillon1000/react

Fork ReactFiberLane (#20371)

This wasn't forked previously because Lane and associated types are
opaque, and they leak into non-reconciler packages. So forking the type
would also require forking all those other packages.

But I really want to use the reconciler fork infra for lanes changes.
So I made them no longer opaque.

Another possible solution would be to add separate `new` and `old`
fields to the Fiber type, like I did when migrating from expiration
times. But that seems so excessive. This seems fine.

But we should still treat them like they're opaque and only do lanes
manipulation in the ReactFiberLane module. At least until the model
stabilizes more. We'll just need to enforce this with discipline
instead of with the type system.