intl: Migrate off of synthetic package; check-in generated files

`flutter` tool deprecated the `package:flutter_gen` synthetic package:
  https://flutter.dev/to/flutter-gen-deprecation

So, instead migrate to referencing source files directly.

Without this change, `flutter run` generates the following warning:

  Synthetic package output (package:flutter_gen) is deprecated:
  https://flutter.dev/to/flutter-gen-deprecation. In a future
  release, synthetic-package will default to `false` and will
  later be removed entirely.

Author: rajveermalviya

l10n.yaml

@@ -1,7 +1,9 @@
 # Docs on this config file:
 #   https://docs.flutter.dev/ui/accessibility-and-localization/internationalization#configuring-the-l10nyaml-file
 
+synthetic-package: false
 arb-dir: assets/l10n
+output-dir: lib/generated/l10n
 template-arb-file: app_en.arb
 required-resource-attributes: true
 output-localization-file: zulip_localizations.dart