nano.spec: Introduce default-editor package
    
    The package is needed for the smooth upgrade process if you don't have
    nano as a default editor.
    
    Currently 'dnf upgrade' to newer Fedoras ends with error because
    'nano-default-editor' is in DNF group 'Standard', which DNF wants to
    install/upgrade during upgrade process. This ends in conflict between
    'nano-default-editor' and any other 'system-default-editor' providers,
    such as 'vim-default-editor' and manuall intervention is needed.
    
    To fix the behavior we introduce 'default-editor' subpackage, which will
    have a weak dependency on 'nano-default-editor', so fresh installations
    will have 'nano' as default, and non-nano users will have an upgrade
    process without this error.
    
    Requiring system-default-editor was added based on pkratoch's review.
    Now default-editor is removed if nano-default-editor is removed and if
    nano-default-editor is excluded from a DNF transaction which installs
    default-editor, DNF will satisfy the dependency with another package
    which provides system-default-editor.