Make `AnimationNodeExtension` extend `AnimationNode` instead of `AnimationRootNode`

[TokageItLab]

• Folow up #99181

Context https://github.com/godotengine/godot/pull/99181#discussion_r1885392444

we will need two (or three) Extensions, AnimationRootNodeExtension and AnimationNodeSyncExtension (and AnimationNodeExtention). If it is not an Extension by replacement, then only the AnimationNode needs to be replaced.

This PR makes it no longer allow the AnimationNodeExtention to be placed "directly" in the StateMachine as an Extension, but provides a workaround to place it by wrapping it in a NodeBlendTree.

Perhaps this class should be deprecated in the future and this should all be done in the AnimationNode after implementing GDScript struct.

[GuilhermeGSousa]

Do you want me to add extension for Root and Sync nodes as well?

[TokageItLab]

I don't know how much demand there is for that. As I wrote in description, RootNode can be solved by wrapping with BlendTree, and Sync may not really be that important as well since it only adds a property that does nothing by itself.

[akien-mga]

Thanks!