Ignore mutations that is not under editor

[JiuqingSong]

in textMutationObserver, we will go through mutations that is not made by content model and see if we can reconcile the change and reflect it in existing cached Content Model. And if an mutation happens under an entity, we should ignore it.

However, it is possible that we delete an entity node from editor and modify some node under this deleted node. Now when we handle this mutation, since the target node is already removed, existing code will return false when we check if it is under an entity. This causes the mutation won't be ignored and finally an existing cache will be invalidated.

Fix: Check if the target node is under editor. If so, directly ignore this change. So cache can be preserved.