complete brightnessctl flags

[llenck]

Description

This adds completions for all of brightnessctls flags. The completed device classes are taken from the source.

I don't use LC_ALL=C for the invocation that lists device names, as -m enables "machine-readable" output that I expect to be stable across languages.

TODOs:

• n/a Changes to fish usage are reflected in user documentation/manpages.
• n/a Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst