Add Inventory system with Inventory, InventorySlot, and StatModifierItem classes; update character attributes to include level and experience

Assets/Scripts/Inventory/StatModifierItem.cs

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+using System.Collections.Generic;
+using Interfaces;
+using Sirenix.OdinInspector;
+using Sirenix.Serialization;
+using UnityEngine;
+
+namespace Inventory
+{
+    [CreateAssetMenu(menuName = "Game/Item/StatModifierItem")]
+    public class StatModifierItem : SerializedScriptableObject
+    {
+        public string itemName;
+        [TextArea] public string description;
+        public Sprite icon;
+        
+        public List<IStatModifier> cures = new();
+        public List<IStatModifier> curses = new();
+    }
+}