Allow customization of current scene position in SceneTree

[timothyqiu]

In one of my projects, every scene is inside a second layer of ViewportContainer & Viewport to achieve retro look 3D scene while keep the UI expandable instead of zoomed.

root (SceneTree.root)
    autoloads
    CustomViewportContainer (SceneTree.current_scene)
        CustomViewport
            actual_scene_content

SceneTree.current_scene is always added as a child of root. It would be better if I can change where it is located, e.g. by overriding a virtual method SceneTree._add_current_scene(node).

root (SceneTree.root)
    autoloads
    CustomViewportContainer (added in custom SceneTree._init)
        CustomViewport (added in custom SceneTree._init)
            actual_scene_content (SceneTree.current_scene)

[TML233]

@godotengine/godot#97914 can solve this.