Add string mutable variable reference

Author: Fireball19

Packages/SOVariables/Runtime/Mutable/Reference/StringMutableVariableReference.cs

@@ -0,0 +1,12 @@
+using UnityEngine;
+
+namespace RaCoding.Variables
+{
+    [System.Serializable]
+    public class StringMutableVariableReference : MutableVariableReference<string>
+    {
+        [SerializeField] private StringMutableVariable stringMutableVariable;
+
+        protected override MutableVariable<string> Reference => stringMutableVariable;
+    }
+}