Build an ES6 module as well

[adroitwhiz]

This is useful for people who are including filesize.js in their own projects using Rollup or some other bundler which supports ES6 modules.

I opted against transpilation with Babel--the use case for this is either that someone is using a bundler (and hence they can add their own transpilation step) or using it directly in a web browser which supports ES6 modules and hence also supports modern JS. As a consequence, this may be a breaking change (if package consumers are using module-aware bundlers, those bundlers will switch over to the non-transpiled module version by default). If desired, I can change it so that the default module is transpiled as well.

[avoidwork]

Thanks for the PR! I prefer what you did here from a packaging pov; this'll be a major bump due to the potential issue... and then we wait & see.