Add command line autocompletion

[joaander]

Description

Support command line autocomplete.

Proposed solution

I'm not sure how other tools do this as I do not use custom autocomplete myself. We should investigate other Rust tools that support autocomplete and see how they handle the situation.
clap-complete may help us generate the completions.

Additional context

I'm not sure how tools do this in general. We could generate files (one for each supported shell) at compile time. It is not clear how those should be installed into a location where the user's shell will look for them. The other option is to add a sub-command row show autocomplete that generates the completion at runtime.

If it is not possible to install files into the shell's completion search patch, we may need to instruct users to set an appropriate environment variable or execute source $(row show autocomplete).

At a minimum, we should support:

• bash
• fish
• zsh

[joaander]

#52 makes it more difficult to autocomplete directory names (or any workspace-specific quantity). How would the user select which workspace to autocomplete from?