Add healing effects and systems with event handling

GameCore/Events/HealDealtEvent.cs

@@ -0,0 +1,10 @@
+using GameCore.ECS;
+using GameCore.Events.Interfaces;
+
+namespace GameCore.Events;
+
+public readonly struct HealDealtEvent(Entity target, float amount) : IEvent
+{
+    public readonly Entity Target = target;
+    public readonly float Amount = amount;
+}