Integrate completions into the homebrew completions directory

[aoberoi]

I recently ran into an issue where I wasn't able to get op completions to work. It turns out that the line I'm supposed to add to my .zshrc (eval "$(op completion zsh)"; compdef _op op) was too early, and needed to be placed after the call to compinit.

However, homebrew has a convention for packages to offer shell completions. Using this convention would eliminate the step where users have to add the line to their .zshrc (or equivalent shell dotfile). As far as I understand, the formula should use the generate_completions_from_executable() method in the formula. It seems like it would be pretty easy to integrate, given the op command can output its completions already. Here's an arbitrary example from the caddy formula.

I think this would remove the trickiest step from the install and make it so much cleaner. If this is something you'd take a contribution for, let me know, and I can send a PR.