Improve async/await usage, and sort init calls in index.js (#17386)

* clean up async/await, and sort init calls in `index.js
* use `const _promise` to indicate that we do not need await an async function

web_src/js/features/dropzone.js

@@ -3,7 +3,6 @@ export default async function createDropzone(el, opts) {
     import(/* webpackChunkName: "dropzone" */'dropzone'),
     import(/* webpackChunkName: "dropzone" */'dropzone/dist/dropzone.css'),
   ]);
-
   Dropzone.autoDiscover = false;
   return new Dropzone(el, opts);
 }