Add support for XPM. (#3706)

* .xpm and .pm extensions associated with XPM.

* .pm is disambiguated by searching the /* XPM */ string.
  This is how `file` performs detection and should work with
  every XPM3 file (most XPM generated by software later than 1991).

Added XPM samples:

* stick-unfocus.xpm: extracted from Fluxbox (MIT License)
  https://github.com/fluxbox/fluxbox/blob/0c13ddc0c85ed4445ff8e289387c142e0ac98c77/data/styles/Emerge/pixmaps/stick-unfocus.xpm

* cc-public_domain_mark_white.pm: public domain image from
  https://commons.wikimedia.org/wiki/File:Cc-public_domain_mark_white.svg
  converted to XPM with ImageMagick (convert input.svg output.xpm).

lib/linguist/heuristics.rb

@@ -349,6 +349,8 @@ module Linguist
         Language["Perl 6"]
       elsif /\buse\s+(?:strict\b|v?5\.)/.match(data)
         Language["Perl"]
+      elsif /^\s*\/\* XPM \*\//.match(data)
+        Language["XPM"]
       end
     end
 

lib/linguist/languages.yml

@@ -4891,6 +4891,14 @@ XML:
   - Web.config
   - packages.config
   language_id: 399
+XPM:
+  type: data
+  extensions:
+  - ".xpm"
+  - ".pm"
+  ace_mode: c_cpp
+  tm_scope: source.c
+  language_id: 781846279
 XPages:
   type: programming
   extensions: