Make "convert to subgraph" undoable (doesn't delete the sub graph though)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphEditWindow.cs

             if (path.Length == 0)
                 return;
 
+            graphObject.RegisterCompleteObjectUndo("Convert To Subgraph");
             var graphView = graphEditorView.graphView;
 
             var nodes = graphView.selection.OfType<MaterialNodeView>().Where(x => !(x.node is PropertyNode)).Select(x => x.node as INode).ToArray();