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

Assets/Scripts/Inventory/InventorySlot.cs

@@ -0,0 +1,13 @@
+using System;
+using Sirenix.Serialization;
+using UnityEngine;
+
+namespace Inventory
+{
+    [Serializable]
+    public class InventorySlot
+    {
+        [OdinSerialize] public ScriptableObject item;
+        [OdinSerialize] public int quantity;
+    }
+}