Add healing effects and systems with event handling

GameCore/Combat/Effects/HealEffect.cs

@@ -0,0 +1,14 @@
+using GameCore.Combat.Interfaces;
+using GameCore.Events;
+
+namespace GameCore.Combat.Effects;
+
+public class HealEffect(float amount) : IEffect
+{
+    public void Execute(EffectContext context)
+    {
+        if (context.Target == null) return;
+
+        context.World.PublishEvent(new HealDealtEvent(context.Target.Value, amount));
+    }
+}