Add a better Hack grammar support

[azjezz]

see #4466

Description

Checklist:

• I am changing the source of a syntax highlighting grammar

  • Old: https://github-lightshow.herokuapp.com/?utf8=%E2%9C%93&scope=text.html.php&grammar_format=json&grammar_url=&grammar_text=&code_source=from-text&code_url=&code=namespace+GitHub%5CLinguist%3B%0D%0A%0D%0Ause+namespace+HH%5CLib%5CSecureRandom%3B%0D%0A%0D%0Aasync+function+foo%28%29%3A+Awaitable%3C%3Fstring%3E+%7B%0D%0A++%24bar+%3D+SecureRandom%5Cint%280%2C+10%29%3B%0D%0A++if+%28%24bar+%3E+8%29+%7B%0D%0A++++return+%27baz%27%3B%0D%0A++%7D%0D%0A%0D%0A++return+null%3B%0D%0A%7D

  • New: https://github-lightshow.herokuapp.com/?utf8=%E2%9C%93&scope=from-url&grammar_format=json&grammar_url=https%3A%2F%2Fgithub.com%2Fslackhq%2Fvscode-hack%2Fblob%2Fmaster%2Fsyntaxes%2Fhack.json&grammar_text=&code_source=from-text&code_url=&code=namespace+GitHub%5CLinguist%3B%0D%0A%0D%0Ause+namespace+HH%5CLib%5CSecureRandom%3B%0D%0A%0D%0Aasync+function+foo%28%29%3A+Awaitable%3C%3Fstring%3E+%7B%0D%0A++%24bar+%3D+SecureRandom%5Cint%280%2C+10%29%3B%0D%0A++if+%28%24bar+%3E+8%29+%7B%0D%0A++++return+%27baz%27%3B%0D%0A++%7D%0D%0A%0D%0A++return+null%3B%0D%0A%7D

[pchaigno]

Thanks for the pull request! LGTM!

[lildude]

Squeezed in in the nick of time for inclusion in v7.5.0 (delayed due to hitting a niggle with the mime-types).