How to update the ICU4X version used in package:intl4x.

First PR

1. Create PR.
2. Update submodules/icu4x to whatever branch/hash you want.
3. Land PR.
4. Tag with intl4x-icu*, push. This creates new release with the new binaries.

Second PR

1. Create PR.
2. Run bash tools/regenerate_bindings.sh, and fix resulting errors.
3. Update const version in version.dart to tag.
4. Regenerate hashes using cd pkgs/intl4x/; dart --enable-experiment=native-assets run tool/regenerate_hashes.dart.
5. Land PR.