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

Assets/Scripts/Inventory/WeaponItem.cs

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+using Sirenix.Serialization;
+using UnityEngine;
+
+namespace Inventory
+{
+    [CreateAssetMenu(menuName = "Game/Item/WeaponItem")]
+    public class WeaponItem : ScriptableObject
+    {
+        [OdinSerialize] public string weaponName;
+        [OdinSerialize, TextArea] public string description;
+        [OdinSerialize] public GameObject prefab;
+        [OdinSerialize] public Sprite icon;
+    }
+}