main/hunspell-en: split into per-dialect subpackages

This package currently provides all dictionaries for all English dialects/variants. When installing it, applications will detect all eleven dictionaries and show all of them as an available dictionaries.

This hurts usability substantially; in Firefox I'd expect to see four dictionaries to quickly chose between the languages that I use, but I end up seen over a dozen, ten of which are unused English variants. At a glance, these are also quite similar, so picking for this large menu is extra irritating.

Split all variants into separate subpackages, to allow the installation of just the desired dictionaries while skipping the others.

The hunspell-en package is now a metapackage that provides all these new subpackages. This avoid breaking any existing installations as this package continues providing the same functionality as before.