Add link hook to package:intl4x

[mosuem]

Download datagen and data postcard files, and use them to link the static libraries.

This needs the new data asset loading feature to load the .postcard data, as well as record_use to tree-shake unused symbols from the library.

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PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

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Package	Changed Files

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

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// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
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Files
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Package publish validation ✔️

Package	Version	Status
package:intl	0.20.0-wip	WIP (no publish necessary)
package:intl_translation	0.20.1-wip	WIP (no publish necessary)
package:messages	0.2.0	already published at pub.dev
package:messages_builder	0.2.1	already published at pub.dev
package:messages_serializer	0.2.1	already published at pub.dev
package:messages_shrinker	0.2.2-wip	WIP (no publish necessary)

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