Add healing effects and systems with event handling

GameCore/Events/EntityHealedEvent.cs

@@ -0,0 +1,11 @@
+using GameCore.ECS;
+using GameCore.Events.Interfaces;
+
+namespace GameCore.Events;
+
+public readonly struct EntityHealedEvent(Entity target, float newHealth, float amountHealed) : IEvent
+{
+    public readonly Entity Target = target;
+    public readonly float NewHealth = newHealth;
+    public readonly float AmountHealed = amountHealed;
+}