feat(gerrit): Syntax highlight rules.pl as Prolog.

This adds support for overriding the detected languages based on the
filename - we assume here that rules.pl will always map to a Prolog
file.

I could've overridden the entire default language to Prolog, since it's
unlikely that we'll have any Perl here, but given the relative
popularity of the two languages I opted to just override the file we
know we'll have (because it's used by Gerrit itself).

https://usercontent.irccloud-cdn.com/file/yhZZx1nd/highlighted_prolog.png

Change-Id: I26a7e6dab191e0b80a027b026f884020a1f07178
Reviewed-on: https://cl.tvl.fyi/c/depot/+/254
Reviewed-by: tazjin <mail@tazj.in>

third_party/gerrit/default.nix

@@ -42,6 +42,7 @@ pkgs.buildBazelPackage {
   patches = [
     ./use_detzip.patch
     ./syntax_highlight_nix.patch
+    ./syntax_highlight_rules_pl.patch
   ];
 
   bazelTarget = "release";