ESM

[brettz9]

This builds on #410 in such a way as to use Rollup instead of browserify. I used browserify in that PR as it meant there were fewer changes necessary, but my intent was always to provide this here as ESM.

These are breaking changes as a result of exports (and if you're ok with the different paths I set up).

If the number of changes here are manageable, I can just close #410 , and if not, I can rebase this on #410 if it may be acceptable.

A few items to mention...

1. In order to reduce diff noise in the PR, I didn't remove the IIFE indent in the main file. If you like I can do that as a separate PR (and which should be easy to confirm, as we can let ESLint auto-fix that.
2. I don't imagine there is a need any longer for a browser export, as I've switched to separate entry points for browser and Node. I also haven't included a version export. Let me know if you want to remove these or ensure they still exist.
3. I changed the exported paths to dist if that is all right; haven't updated the README yet with the new paths, as I wasn't sure you would be ok with them.