just-completions: support module completions

[orthonormalremy]

A pretty self-explanatory change. Here's what it looks like:

~/repos/nu_scripts/custom-completions/just> just --version
just 1.43.1
~/repos/nu_scripts/custom-completions/just> nu --version
0.108.0
~/repos/nu_scripts/custom-completions/just> # how the completions look
~/repos/nu_scripts/custom-completions/just| just                                                                                                                                                                                                                               12/01/2025 12:20:43 AM
a-regular-top-level-recipe
amzn::ada
amzn::update_burner_accts
dotfiles
dotfiles::change-profile
dotfiles::format
dotfiles::stow-chkstow
ec2::ensure-primary-firewall
ec2::ls_instances

[fdncred]

ok, let's try it. thanks