Add fish shell completions to brew

[lwolfsonkin]

Closes #6172

• Copy fish completions from github.com/laughedelic/brew-completions
• Update completions readme to include fish
• Rename fish functions to be more styled like most fish functions
• Make completions for brew pin suggest only unpinned formulae
• Added TODO for dropping brew search's cache desc_cache.json
• Drop completions for nonexistent brew cask cleanup

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• Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

• Copy fish completions from github.com/laughdelic/brew-completions

This URL doesn't work; can you confirm the source and if the license permits this?

Otherwise: looks good 👍

[erikgaal]

The correct URL should be https://github.com/laughedelic/brew-completions/.

The license is GPLv3 so I don't think we can just merge this.

[reitermarkus]

laughedelic/brew-completions#9

[MikeMcQuaid]

@laughedelic can you confirm you are happy for your code to be included in this repository under these terms: https://github.com/Homebrew/brew/blob/master/LICENSE.txt?

[lwolfsonkin]

@MikeMcQuaid: I corrected the URL in the PR description now. :)

[laughedelic]

Thank you @lwolfsonkin for pushing this forward and taking extra effort for the improvements! 👏

I'm fine with the BSD license, but I would appreciate being mentioned as a co-author in the merge commit if this is fine with you:

Co-authored-by: Alexey Alekhin <[email protected]>

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @lwolfsonkin!

I'm fine with the BSD license, but I would appreciate being mentioned as a co-author in the merge commit if this is fine with you:

Done, thanks for your initial work on this too @laughedelic: you also rock!