Prior to v89, Firefox has bugs with document.execCommand("insertText"): https://bugzil.la/1220696
This commit introduces some variables to browser-detect, and therefore wraps the entire logic in an IIFE to avoid state leaking. (`let`/`const` are not supported on older browsers)
All files under `app/assets/javascripts/scripts` will be run through babel, given sourcemaps, and output under `/assets/{name}.js`. This is another step towards removing our sprockets dependence.
