/// Generates shell completion scripts for frostx.
///
/// Usage: `gen_completions` [`OUTPUT_DIR`]
///
/// Writes `frostx.bash`, `_frostx` (zsh), `frostx.fish`, and `frostx.elv`
/// (elvish) to `OUTPUT_DIR`, defaulting to `./completions` when omitted.
use CommandFactory;
use Shell;
use PathBuf;