Adapt package:messages to be loading mechanism independent

[mosuem]

• Stop using build_runner in favor of dart run messages
• Make loading injectable again, to be independent of the progress of dart:asset
• Add Flutter examples
• Fix a few small bugs (such as zero not being parsed)

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[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
messages_shrinker	Non-Breaking	0.2.2-wip	0.2.2-wip	0.2.2-wip	✔️

Changelog Entry ✔️

Package	Changed Files

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

Coverage ✔️

File	Coverage
pkgs/messages_shrinker/lib/messages_shrinker.dart	💚 90 %

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pkgs/messages/examples_flutter/my_application/lib/src/empty_de.dart
pkgs/messages/examples_flutter/my_application/lib/src/empty_en.dart

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[devoncarew]

Just a few thoughts -