shell completion installs

[cwlbraa]

requires #130

this PR installs shell completions using homebrew post install hooks and provides instructions on how to install them as install.sh output.

complexities:

• bash: there are canonical locations for bash, but those locations require the (very standard) bash-completion plugin.
  • in homebrew, we'll just assume you have that package or if you're using bash over default zsh, you'll guess that you need it.
• zsh: there are no canonical locations, but you can look up locations from fpath if your shell is all configured (ohmyzsh, antigen, etc).
  • homebrew documents how things need to be set up for zsh completions, and so long as the user has eval "$(brew shellenv)" in their zshrc, we can write to $(brew --prefix)/share/zsh/site-functions pretty blindly
• fish is friendly so it's got canonical paths 🎉

because it's complex to detect what zsh/bash setups a given installer has from inside the installer script, we instead have the install script tell users both the "dumb" rc-centric way to install, and the most common "managed" way.

one alternative approach that folks have talked about is putting this in a cu init subcommand or similar. we could do that, but it has all the same difficulties that come with putting this in autoinstalling from our bash-specific install script. there are too many possible shells, userspace package managers, and plugin frameworks and custom configurations (including the absence of them).

NOTE: i haven't tested the homebrew tap, don't merge this without testing it on a fork

[tiborvass]

Docker adds in zshrc, an fpath as the first entry. But i don't want to add cu's completion to my docker's completion folder.

[cwlbraa]

🤔 under some package managers fpath[1] isn't gonna be user-writable either, so unideal in several situations.

can i get a citation for the docker approach?

[cwlbraa]

tested homebrew on my fork https://github.com/cwlbraa/homebrew-tap/commits/main/, working correctly and installing cu completions at least for zsh.

if you've already got completions in that brew directory, it throws this:

Error: It seems there is already a Zsh Completion at '/opt/homebrew/share/zsh/site-functions/_cu'.

but after deleting that file and reinstalling the error goes away.