add: LoadingScene, ProxyNode

src/Scenes/RootScene.cs

@@ -1,6 +1,8 @@
 using Godot;
+using Polonium.Attributes;
+
 namespace Polonium.Scenes;
-[GlobalClass]
+[ProxyNode]
 public partial class RootScene : Node2D
 {
     private Scene CurrentScene { get; set; }