Implements https://github.com/godotengine/godot-proposals/issues/4096
* Nodes can be marked unique to the scene in the editor (or via code).
* Unique nodes can be accessed via the **%** prefix at any point in the path. From that point in the path (depending on whether the scene of the path is), the unique node will be fetched.
* Implementation is very optimal, as these nodes are cached.
