dillon1000/react

Add separate ChildDeletion flag (#20264)

In the old, effect list implementation, the Deletion flag is is set on
each deleted fiber.

In the new, subtreeTag implementation, the Deletion flag is set on the
parent of each deleted fiber, and the deleted fibers themselves are
pushed to the `deletions` array.

To better distinguish between these two uses, I've added a separate
ChildDeletion flag. That way we can, if desired, maintain both
implementations simultaneously, as we bisect to find the performance
regression that we're currently investigating.