Add shell completions

[scaryrawr]

Describe the feature or problem you'd like to solve

Add support for generating tab completion for shells

Proposed solution

Many tools have some command like:

copilot completion zsh

Where it will output completions for integration with the user's shell, so in .zshrc or powershell $PROFILE you could add something like:

eval "$(copilot completion zsh)"

And get tab completion for things like:

copilot --al # hit tab to complete --allow-all-tools

Example prompts or workflows

No response

Additional context

No response

[EvanBoyle]

Agreed, we definitely need to do this. Will raise it for the next planning cycle.