Add Node script to generate the identifier regexes

The script was taken from https://gist.github.com/mathiasbynens/6334847 and then modified to match the needs of this project.

This way, it’s easy to update the regexes when a new Unicode version is released, or when you want to match ES6 instead of ES6 (https://gist.github.com/mathiasbynens/6334847 already has the code for that).