Add support for transpiling ES6 Unicode regular expressions

**Do not merge this yet!**

@arv This is an initial draft of how I think this could work. The build fails for obvious reasons (see below) but do you have any feedback on the general implementation for now?

How do you want to pull in the regexpu dependency? I’m using `require()` as an example in this patch. Let me know if you want me to make any changes to regexpu’s export logic.
So there would be no way of using the npm version directly, right? We’d have to put the entire ES6 regexpu source code in this repository and manually keep it in sync with the upstream repo. Is that correct? Is there a better way?