Add API so that we can find the type of subshaders that can be added.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Nodes/MasterNode.cs

         {
             get { return PreviewMode.Preview3D; }
         }
+
+        public Type supportedSubshaderType
+        {
+            get { return typeof(T); }
+        }
         
         public IEnumerable<T> subShaders
         {