Add initial implementation of game mechanics and resources management

Scripts/Core/TierDefinition.cs

@@ -0,0 +1,10 @@
+using Godot;
+
+namespace ParasiticGod.Scripts.Core;
+
+[GlobalClass]
+public partial class TierDefinition : Resource
+{
+    [Export] public PackedScene Scene { get; private set; }
+    [Export] public long Threshold { get; private set; }
+}