Add game scene and level catalog interfaces, and implement scene management in AppRoot

app/IGameScenes.cs

@@ -0,0 +1,10 @@
+using Godot;
+
+namespace Mr.BrickAdventures.app;
+
+public interface IGameScenes
+{
+    void Load(PackedScene scene);
+    void Restart();
+    void ReturnToMain(PackedScene mainMenu);
+}