Add Rollup and node-sass to build tooling; produce css and minified CSS plus an ES6 bundle and IIFE bundle with Rollup.

Created by: danielnaab

The ES6 bundle lets us import 'wdfn-viz without using the Rollup CommonJS plugin. Also added module and sass, and css entries to package.json for tooling that looks for those properties.

I'm testing locally against the Hugo theme, until we can get this published.