Add necessary typecast for supporting jlab 1 and 2

Author: jasongrout

{{cookiecutter.github_project_name}}/src/plugin.ts

@@ -29,7 +29,9 @@ const examplePlugin: IPlugin<Application<Widget>, void> = {
   requires: [IJupyterWidgetRegistry],
   activate: activateWidgetExtension,
   autoStart: true
-};
+} as unknown as IPlugin<Application<Widget>, void>;
+// the "as unknown as ..." typecast above is solely to support JupyterLab 1
+// and 2 in the same codebase and should be removed when we migrate to Lumino.
 
 export default examplePlugin;