Add programs.borgmatic.backups.<name>.settings for native borgmatic
YAML and generate each backup file from it.
The location, storage, retention, and consistency options and the
section extraConfig options become deprecated aliases into settings.
Warnings name the affected backup. Legacy configurations without
overlapping keys produce the same YAML, and Home Manager symlink
exclusions still append to exclude_from.
Drop the assertions that rejected backups setting both or neither of
sourceDirectories and patterns, so native source patterns and
database-only backups work.
