Add missing tools/generate-identifier-regex.js

[mathiasbynens]

babylon/src/util/identifier.js

Line 37 in a1f42e7

// Generated by `tools/generate-identifier-regex.js`.

mentions tools/generate-identifier-regex.js but no such script exists in the repo.

It appears this was copied from https://github.com/ternjs/acorn/blob/3b4ab11ead6eb5fc4f9d2a8ef535d3b2ce699af1/src/identifier.js#L35.

Either way, please include the script.

[forivall]

The original is here: https://github.com/ternjs/acorn/blob/master/bin/generate-identifier-regex.js

I also have a slightly modified version at https://github.com/forivall/tacoscript/blob/master/packages/horchata/bin/generate-identifier-regex.js