Implement health modification on enemy kill; add HealOnKillModifier and update death behavior

Assets/Scripts/Modifiers/HealOnKillModifier.cs

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+using System;
+using Data;
+using Interfaces;
+using Systems;
+using UnityEngine;
+
+namespace Modifiers
+{
+    [Serializable]
+    public class HealOnKillModifier : IStatModifier, IOnKillEffect
+    {
+        public float value;
+        public string Description => $"+{value} Health on Kill";
+        
+        public void Apply(CharacterAttributes attributes) { }
+
+        public void Remove(CharacterAttributes attributes) { }
+        
+        public void OnKill(GameObject killer, GameObject victim)
+        {
+            killer.TryGetComponent(out Character character);
+            character?.attributes.ModifyHealth(value);
+        }
+    }
+}

Assets/Scripts/Modifiers/HealOnKillModifier.cs.meta

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+fileFormatVersion: 2
+guid: b329a84895234a3290156c5afe257212
+timeCreated: 1752314314