Accomodate Nuspell
    
    Given that itʼs compatible with Hunspell dictionaries, package `nuspell` requires `hunspell-en-US`.  It doesnʼt seem right that installing one spellchecking backend should cause a different one to be pulled in, so donʼt depend directly on `hunspell`.
    
    Instead, recommend at least one of `hunspell` or `nuspell`.
    
    This commit depends on a companion commit to `hunspell` to create the new `hunspell-filesystem` package to own the `/usr/share/myspell` directory.