Use rollup.js instead of grunt/browserify/etc

[mallorydxw]

@erbridge recommended using this

https://rollupjs.org/guide/en/

[anthonyomalley]

On thing missing from Wilton Park was https://github.com/gruntjs/grunt-contrib-watch which allows code to be compiled and the browser to be reloaded automatically when a change is made within certain folders (e.g. the scss folder).

It looks like https://www.npmjs.com/package/rollup-plugin-livereload is the equivalent rollup.js plugin.

[mallorydxw]

Another requirement would be to rename the output files when they change (see also #12).

[erbridge]

You could also consider Parcel.