fix: zsh completions for old cask names

[tiluckdave]

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Fixes #20056

Made a new file in the cache_dir containing old cask names as well, so as not to disturb the brew casks command. Updated the completions logic to take it from the new file if it exists, or else defaults to brew casks.

[MikeMcQuaid]

Thanks for the PR @tiluckdave!

@Homebrew/maintainers interested in thoughts on this. I'd rather we didn't write another file here but I don't think it makes for brew casks (or even completions, honestly) to handle the old names. Thoughts?

[EricFromCanada]

If such a feature is to be added, it should be done for bash/fish/zsh and for both formulae and casks. Also, instead of generating a new file with cask's current and old names mixed together, why not implement something similar to how formula_aliases.txt is generated and read? Also, is it possible for tab completion to auto-correct to the new name when completing the old name?

[tiluckdave]

@EricFromCanada @MikeMcQuaid

I can update this for formulae as well as on bash/zsh/fish. But first, let me explore whether we can autocomplete to a new name. If I find something, I will share my approach here.