l10n Localizations and LocalizationsDelegate Generation Tool (#43934)

*Starting with arb files in lib/l10n, the tool creates the following by default:
1. An AppLocalizations class containing:
- an `AppLocalizationsDelegate`
- a `supportedLocales` property that returns a list of all supported locales.
- a `localizationsDelegate` property that returns a list containing this delegate along with `GlobalMaterialLocalizations.delegate`, `GlobalCupertinoLocalizations.delegate`, and `GlobalWidgetsLocalizations.delegate`.
2. One Dart `message_<locale>.dart` file for each arb file, as well as a `messages_all.dart` file that performs the locale message lookup. The tool infers the locale of each arb file from the @@Locale property or the arb file's name.