Redesign and reimplement inventory menu

Add jewels but no implementation yet (needed redesign of inventory menu to function correctly)

Zennysoft.Game.Godot.Implementation/Entity/IStackable.cs

@@ -0,0 +1,10 @@
+using Chickensoft.Collections;
+
+namespace Zennysoft.Game.Implementation;
+
+public interface IStackable
+{
+  AutoProp<int> Count { get; }
+
+  void SetCount(int count);
+}