Add a subcommand that prints out a space-separated list of the names of
top-level variables in the justfile.
The syntax is:
$ just --variables
a b c
This can be used for any purpose, but is mostly intended for completion
scripts, so that they can get the names of variables without using
`--evaluate`.
Additionally:
- Add `bin/generate-completions` script to regenerate checked-in
completions
- Update dependencies
- Regenerate checked-in completions
