Generate zsh completions automatically

[Rylan12]

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Continuation of the work for #10223.

This PR adds the ability for brew man to automatically generate zsh completions. Like with the bash completions, this isn't super intelligent at the moment. It doesn't know how to handle conflicting options or anything like that.

One thing that's different for zsh compared to bash completions is that zsh completions allow for descriptions to be used. Descriptions can be used for the brew commands themselves as well as for the options.

To automatically generate the descriptions for options, the description parameter defined for the option is used.

To generate descriptions for commands, I chose to use the first sentence of the command description. I skimmed through the generated descriptions to make sure they seemed reasonable but didn't look that closely. The way the first sentence is determined is by splitting the description by periods. There were a few cases where the first period was still within the first sentence of the description (when using i.e./e.g. or referring to a file with a dot in the path). I modified those command descriptions to not use periods within the first sentence.

[BrewTestBot]

Review period will end on 2021-01-26 at 00:00:00 UTC.

[Rylan12]

One other thing I noticed is that the completions.rb file is starting to fill up. At the moment it handles linking/unlinking third-party completions as well as generating completions for two shells (soon to be a third). Each shell seems to need at least two helper methods plus a constant that maps named_arg types to functions.

It may be a good idea to split this up a little bit. One option is to have something like a CompletionsLinker and CompletionsGenerator module. Additionally or alternatively, the generation for the individual shells could be handled in separate files as well. I know we do something kind of similar with extend/os (the difference being that we only want to use one of the os extensions while we'd always want to use all of the shell completions extensions).

Thoughts on the best thing to do here?

[Rylan12]

Okay, good news! I have fish completions working locally as well. Those require the changes in this PR but I'm going to hold off on pushing those, for now, to help reduce the size of this (already pretty big) PR.

[MikeMcQuaid]

Additionally or alternatively, the generation for the individual shells could be handled in separate files as well.

This sounds good to me! Doesn't have to block this PR, though (in fact it may be easier to split up after this is merged). Good thinking about this 🎉

[Rylan12]

This will be available to merge later today (once the conflicts are addressed). Should I go ahead and merge or would you rather I wait until after #10397 is merged per #10397 (review). Not sure what your estimated timing on that one is. Whichever is merged second will need to be rebased. It doesn't matter to me which one it is.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

This will be available to merge later today (once the conflicts are addressed). Should I go ahead and merge or would you rather I wait until after #10397 is merged per #10397 (review). Not sure what your estimated timing on that one is. Whichever is merged second will need to be rebased. It doesn't matter to me which one it is.

Let's ship this first. Thanks again @Rylan12, great work!