Add bash/zsh/fish/... completions on install

[eugenesvk]

Would be nice to have them auto included and updated without much/any user intervention :)

This is similar to wezterm/wezterm#3008, but thought this might be the appropriate place for a dedicated issue

[wez]

This issue needs more information about the system/distribution/environment where you are running into this.

[wez]

Err, scratch that, I thought this was filed in the main wezterm repo!
I'll accept a PR to add this!