Improve type annotation for requireLoader

Author: Madhu94

packages/html-manager/src/libembed-amd.ts

@@ -54,7 +54,7 @@ function requireLoader(moduleName: string, moduleVersion: string) {
  * @param element (default document.documentElement) The element containing widget state and views.
  */
 export
-function renderWidgets(element = document.documentElement, loader: Function) {
+function renderWidgets(element = document.documentElement, loader: (moduleName: string, moduleVersion: string) => Promise<any>  = requireLoader) {
     requirePromise(['@jupyter-widgets/html-manager']).then((htmlmanager) => {
         let managerFactory = () => {
             return new htmlmanager.HTMLManager({loader: loader || requireLoader});