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

app/ILevelCatalog.cs

@@ -0,0 +1,11 @@
+using Godot;
+
+namespace Mr.BrickAdventures.app;
+
+public interface ILevelCatalog
+{
+    int Count { get; }
+    PackedScene? Get(int index);
+    PackedScene First { get; }
+    PackedScene MainMenu { get; }
+}