move field codes to string capture group

Author: gg582

src/languages/freedesktop.js

@@ -16,7 +16,7 @@ export default function(hljs) {
     className: 'string',
     begin: /"/,
     end: /"/,
-    contains: [ hljs.BACKSLASH_ESCAPE ],
+    contains: [ hljs.BACKSLASH_ESCAPE, FIELD_CODES ],
     relevance: 0
   };
 
@@ -52,7 +52,6 @@ export default function(hljs) {
     end: /$/,
     excludeBegin: true,
     contains: [
-      FIELD_CODES,
       STRING,
       {
         className: 'literal',