Add completions for `downgrade` tool
Add new `--installed` option for printing Arch packages
Change Arch Linux package related completions to use `--installed`
add newline
__fish_complete_suffix accepts a first argument containing a
brace-expansion, like
__fish_complete_suffix '.{c,cpp,py}'
We're gonna be removing the `eval` that does that shortly, so let's
remove all uses in our code.