Feature-gated binary (--features completions) that generates bash/zsh/fish/
elvish/powershell completions from each tool's uu_app() definition.
Usage:
shadow-rs-completions passwd --shell bash
shadow-rs-completions --all --shell zsh --dir completions/
Also updates generate-completions.sh to use the new binary and adds
windows-sys skip to deny.toml for transitive clap dep.
Fixes#106