Add package:messages_builder

[mosuem]

Add the code to generate the messages files, together with an example in the package:messages directory.

At the moment, the builder takes more options than we would probably want (some have been pushed to future PRs for easier review). For the stable version of this package, the plan is to cut back on these to reduce future maintenance burden.

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[github-actions]

PR Health

Package publish validation ✔️

Details

Package	Version	Status
package:intl4x	0.7.0	already published at pub.dev
package:intl_translation	0.19.0-dev	pre-release version (no publish necessary)
package:intl	0.19.0-dev	pre-release version (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

License Headers ❗

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// Copyright (c) 2023, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
pkgs/messages/example_json/lib/testarbctx2.g.dart
pkgs/messages/example_json/lib/testarb.g.dart

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/intl_translation/test/generate_localized/code_map_messages_all_locales.dart
pkgs/intl_translation/test/generate_localized/code_map_messages_all.dart
pkgs/intl_translation/test/generate_localized/code_map_messages_fr.dart
pkgs/intl_translation/test/two_components/component_messages_fr_xyz123.dart
pkgs/intl_translation/test/two_components/component_messages_all.dart
pkgs/intl_translation/test/two_components/app_messages_fr.dart
pkgs/intl_translation/test/two_components/app_messages_all.dart
pkgs/intl_translation/test/two_components/component_messages_all_locales.dart
pkgs/intl_translation/test/two_components/app_messages_all_locales.dart
pkgs/intl_translation/example/lib/generated/messages_es.dart
pkgs/intl_translation/example/lib/generated/messages_all_locales.dart
pkgs/intl_translation/example/lib/generated/messages_de.dart
pkgs/intl_translation/example/lib/generated/messages_iw.dart
pkgs/intl_translation/example/lib/generated/messages_all.dart
pkgs/intl_translation/example/lib/generated/messages_de_CH.dart
pkgs/intl_translation/example/lib/generated/messages_en.dart

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Changelog Entry ✔️

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Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

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File	Coverage

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