Support `.arb` Translations that should only be generated for Android or iOS

[phillippbertram]

{
  "android_only_text": "this text will only be genetated for android",
  "@android_only_text": {
     "platform": ["android"]
  }
  "ios_only_text": "this text will only be genetated for ios",
  "@ios_only_text": {
     "platform": ["ios"]
  },
  "all_plattforms_text": "this text be generated for all platforms"
}