Separate script to rebuild msg/json/en.json, qqq.json etc.

There are some files in msg/json/ (currently en.json, qqq.json,
constants.json and synonyms.json) that are generated by
scripts/i18n/js_to_json.py as part of the language file build process
- but this only needs to be done when messages.js is updated and
and usually requires some manual cleanup, so remove this step from the
existing buildLangfiles gulp script and create a separate command
('npm run generate:langfiles') to do this when required.

msg/messages.js

@@ -8,13 +8,15 @@
  * @fileoverview English strings.
  * @author fraser@google.com (Neil Fraser)
  *
- * After modifying this file, either run "build.py" from the parent directory,
- * or run (from this directory):
- * ../scripts/i18n/js_to_json.py
- * to regenerate json/{en,qqq,synonyms}.json.
+ * After modifying this file, run:
+ *
+ *     npm run generate:langfiles
+ *
+ * to regenerate json/{en,qqq,constants,synonyms}.json.
  *
  * To convert all of the json files to .js files, run:
- * ../scripts/i18n/create_messages.py json/*.json
+ *
+ *     npm run build:langfiles
  */
 'use strict';