Add smarter locale filtering in DataExporter

[sffc]

ICU4C's data build tool has a fairly smart algorithm when figuring out which locales to include based on an allowlist: all children and parents. For example, including "en-001" results in all parents ("en", "root") and children ("en-GB", "en-ZA", ...) being included.

We should implement this as well in ICU4X. It should be done in the LanguageIdentifierFilter trait in icu_provider.

This depends on the resolution to #173, since the locale fallback chain will need to be computed by this filter.