Long-term `package:intl`/`intl_translation` strategy

[mosuem]

The long-term plan is to deprecate intl and intl_translation in favor of intl4x and messages. This will hopefully give a better user experience, get rid of bugs, offer more functionality, all while being less of a maintenance burden for the Dart team.

However, this requires the replacement libraries to be stable. Steps to do:

• Have intl4x wrap the Intl object on the web
• Distribute precompiled icu4x binaries via Github releases
• Use build hooks to fetch or build icu4x binaries
• Use link hooks to treeshake icu4x binaries
• Use the new Dart assets feature to load localization data and messages
• Use macros for code generation in messages

[mosuem]

Update: Work on package:messages is paused for now. There are no plans for when this is un-paused.