Support flutter_localizations

[stevemessick]

The new skeleton template includes an example usage of flutter_localizations. In the app_en.arb file is a single definition called appTitle. We need support for Find Usages in the IDE.

I'm not sure if this belongs in the Flutter plugin, the Dart plugin, or the Dart analysis server. I'd guess the analysis server, but it is quite specific to Flutter.

cc @jacob314 @DanTup @alexander-doroshko

[DanTup]

I think it should be in the analysis server too - it's along similar lines to dart-lang/sdk#46722 (which also involves teaching the server about arb files).

[helin24]

Reopening as we expect this would require some work in the IDE in addition to analyzer changes