AddTo support for Godot Node

[minami110]

Add AddTo extension method for Godot.Node class.

public partial class Foo :  Node
{
    private readonly Subject<Unit> _s = new();

    public override void _Ready()
    {
        _s.AddTo(this);
    }
}

Disposable will be disposed when node has been exited tree.
If this method is called before enter tree, there is no guarantee that it will be dispiosed, so a warning log is displayed if it is called on a constructor or the like.

[neuecc]

thanks, I'll check soon!

[neuecc]

I think it's good, I'll merge it!