feat: Add "print completion" command.

[renancaraujo]

Description

Some CLIs choose not to auto-install the completion files but rather to expose a command where a completion script is printed in the stdout.

Then a CLI user's usage would be like this:

my_cli completion-script >> ~/.zshrc

Requirements

• Add this command if and only if auto-install is deactivated

Additional Context

The same approach used by NPM and GH CLI, among others

As described by this comment: #47 (comment)

[bartekpacia]

This would be awesome to have!