Path towards `package:intl4x` 1.0

[mosuem]

Things we need for this package to no longer being experimental:

• Agree on an API which is usable, matches ECMA/ICU4X APIs well enough, and is treeshake-able.
• Agree on a set of locales to provide as the default.
• Test the package through conformance tests, fixing bugs until we are happy.
• build and link hooks in stable
• Usage recording in stable

Things we need for this package to be usable/replace package:intl:

• Review if the binary + data size is small enough. This might mean shipping data assets to support data tree-shaking, or reducing the number of available locales, or improving treeshaking through the linker.
• dart doctor to check if the necessary toolchains are installed, or shipping the necessary toolchains with this, see also Introduce dart doctor & hook/doctor.dart sdk#55117. This is only needed if we require any toolchain on any platform which is not preinstalled.
• Document the package and it's usage extensively.
• Extract most of the build logic to package:icu4x.