This PR:
* During the passive effect complete phase for Offscreen, we add all the transitions that were added to the update queue in the render phase to the transitions set on the memoziedState. We also add the stateNode for the Offscreen component to the root pendingSuspenseBoundaries map if the suspense boundary has gone from shown to fallback. We remove the stateNode if the boundary goes from fallback to shown.
* During the passive effect complete phase for the HostRoot, for each transition that was initiated during this commit, we add a pending transitionStart callback. We also add them to the transition set on the memoizedState for the HostRoot. If the root pendingSuspenseBoundaries is empty, we add a pending transitionComplete callback.
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