warn if some translations-keys are not used anymore

[ivanhofer]

Over time your application will grow. So do your translations. It can happen that you change a feature and add new translations, but forget to remove the old ones.

We could automatically look for unused translations

What needs to be done:

• write a rollup-plugin
  • parse all files for used translation-keys
  • output warning, if a key is present in your translations, but not referenced in your code