dillon1000/react

Bugfix: Don't unmount siblings of deleted node (#19516)

* Test: Don't unmount siblings of deleted node

Adds a failing regression test. Will fix in the next commit.

* Refactor to accept deleted fiber, not child list

A deleted fiber is passed to
flushPassiveUnmountEffectsInsideOfDeletedTree, but the code is written
as if it accepts the first node of a child list. This is likely because
the function was based on similar functions like
`flushPassiveUnmountEffects`, which do accept a child list.

Unfortunately, types don't help here because we use the first node
in the list to represent the whole list, so in both cases, the type
is Fiber.

Might be worth changing the other functions to also accept individual
fibers instead of a child list, to help avoid confusion.

* Add layout effect to regression test, just in case