Use a module loader to offer CJS & Harmony exports for the package.json

Currently we just ship the package and tell people they need to update their babel configs to use this. That is sort of unfair since we use a non-standard feature (classnames) so we should use Webpack, Parcel, or Rollup to bundle and provide more than the current Babel exports.

Rollup has a good read on this: https://github.com/rollup/rollup/wiki/pkg.module