Add syntax highlighting for Perforce (develop a non-GPL grammar for it)

Created by: slimsag

We used to use this syntax definition for highlighting Perforce files before Sourcegraph v3.21, but had to remove it as we discovered it was GPLv2 licensed which would require us to license https://github.com/sourcegraph/syntect_server as GPLv2 as well.

We should develop and open-source grammar for it following what was said in #13933 (closed) - please see that issue if you are interested in taking a shot at this. We are looking for volunteers :)