cask: add generate_completions_from_executable DSL artifact

Adds a new `generate_completions_from_executable` stanza for Casks that
runs an executable at install time to generate shell completion scripts,
mirroring the existing Formula DSL method.

The new `GeneratedCompletion` artifact class supports the same
shell_parameter_format options as Formula (arg, clap, click, cobra,
flag, none, typer, and custom prefix strings).

Fixes #21482.

This contribution was developed with AI assistance (Claude Code + Codex).