whalebrew: add shell completions

[branchvincent]

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Needs #107227 first

[max-ae]

This can use the DSL introduced in Homebrew/brew#13536, otherwise the cop from Homebrew/brew#13553 will fail once #105707 and Homebrew/brew#13553 are merged.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.