Add LESS support (.less)

Cheating slightly as it uses the CSS lexer, as pygments currently does
not have a dedicated less lexer. But I figure language recognition and
90% percent correct syntax highlighting is better than neither.

lib/linguist/languages.yml

@@ -731,6 +731,13 @@ Lasso:
   - .lasso9
   - .ldml
 
+Less:
+  type: markup
+  group: CSS
+  lexer: CSS
+  ace_mode: less
+  primary_extension: .less
+
 LilyPond:
   lexer: Text only
   primary_extension: .ly