intl: Migrate from package:flutter_gen; include generated files

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

The generated localization source files are now included in the
repository and referenced 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

.gitattributes

@@ -3,6 +3,9 @@
 # Dart files generated from the files next to them, or by Pigeon:
 *.g.dart -diff
 
+# Generated file for localizations.
+lib/generated/l10n/*.dart -diff
+
 # Generated files for testing migrations:
 test/model/schemas/*.dart -diff
 test/model/schemas/*.json -diff

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