Add `reparent_to()`

[davcri]

func reparent_node(node: Node2D, new_parent, update_transform = true):
    var previous_xform = node.global_transform
    node.get_parent().remove_child(node)
    new_parent.add_child(node)
    node.global_transform = previous_xform

This is not implemented in Godot because it's easy to do with GDScript.
However it seems like a handy feature to have in a game template (and also to avoid duplicating code).

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Relevant Godot issues:

• https://github.com/godotengine/godot/issues/8788
• https://github.com/godotengine/godot/issues/4008