Add title to PreviewNode

MaterialGraphProject/Assets/UnityShaderEditor/Runtime/Nodes/PreviewNode.cs

     [Title("Preview Node")]
     public class PreviewNode : CodeFunctionNode
     {
-        public const int InputSlotId = 0;
-        public const int OutputSlotId = 1;
-
+
+        public PreviewNode()
+        {
+            name = "Preview";
+        }
 
         protected override MethodInfo GetFunctionToConvert()
         {