Function generate_completion_script 

pub fn generate_completion_script(
    shell: Shell,
    domains: &[(String, Vec<String>)],
    global_flags: &[(&str, Option<char>)],
) -> String

Generate a full completion script for the given shell.

• shell - Target shell.
• domains - (name, aliases) for each domain.
• global_flags - (long_name, optional_short) for global flags.