[less mode] Don't treat @font-face as a variable

Issue #2131

mode/css/css.js

@@ -667,7 +667,7 @@ CodeMirror.defineMode("css", function(config, parserConfig) {
         }
       },
       "@": function(stream) {
-        if (stream.match(/^(media|import)\b/, false)) return false;
+        if (stream.match(/^(font-face|media|import)\b/, false)) return false;
         stream.eatWhile(/[\w\\\-]/);
         if (stream.match(/^\s*:/, false))
           return ["variable-2", "variable-definition"];