This extension uses the Hack language grammar defined in the atom-ide-hack project.
The source file is located at https://github.com/hhvm/atom-ide-hack/blob/master/grammars/hack.cson.
Since it is in CSON format, it has to be converted to JSON before use here. A converter is included as a npm dev dependency, and can be run by:
$ ./node_modules/cson/bin/cson2json syntaxes/hack.cson > syntaxes/hack.json