[new port] Native standalone version of gn build system

[nonameless at ukr.net]

Hi everyone!

Gn, chromium build system, now is standalone project. FreeBSD support was added to source tree recently.

I propose create a new port devel/gn. I created PR with patch - https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234309.

Can anyone look on it?

Thanks.