loadJs static method
It loads the JavaScript file that contains the Js code
Returns: A Future
Implementation
static Future<void> loadJs(String filename) {
  final Completer completer = Completer();
  final scriptUploadLargestFilesLib = ScriptElement();
  scriptUploadLargestFilesLib.type = 'text/javascript';
  scriptUploadLargestFilesLib.onLoad.listen((_) {
    completer.complete();
  });
  /// It's loading the JavaScript file that contains the upload code.
  scriptUploadLargestFilesLib.src =
      '/assets/packages/upload_largest_files/assets/$filename';
  /// It's adding the JavaScript file to the body of the HTML page.
  document.body!.append(scriptUploadLargestFilesLib);
  return completer.future;
}