Meson-ify a few things, scripts, completions, etc. Should make our Meson
build complete except for docs.
Co-Authored-By: Qyriad <qyriad@qyriad.me>
Co-Authored-By: eldritch horrors <pennae@lix.systems>
Allows completing `nix build ~/flake#<Tab>`.
We can implement expansion for `~user` later if needed.
Not using wordexp(3) since that expands way too much.
Make nix output completions in the form `completion\tdescription`.
This can't be used by bash (afaik), but other shells like zsh or fish
can display it along the completion choices
