Add Node script to generate the identifier regexes.

The script was taken from https://gist.github.com/mathiasbynens/6334847
and then modified to match Esprima’s needs.

It replaces the Python script that was previously used for the same
purpose. The new script is simpler, more compact, and doesn’t require
any manual post-processing of the data.

https://github.com/ariya/esprima/pull/199