Load JavaScript as ES6 modules

JavaScript modules have been supported by the spec since ES6 (2015) and
the feature is available in all modern, supported browsers.

Using JavaScript modules offers advantages:

- All scripts are loaded in strict mode, reducing unexpected edge cases
  from "sloppy mode".

- Scripts run in isolation and no longer pollute the global namespace.
  This allows removing the IIFE from all JavaScript.

- Browsers can optimize loading of modules.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

Author: jdufresne