feat: add shell completions

[ShaunSHamilton]

Closes #877

Adds shell completions, by adding a new gix command generate-completions:

gix generate-completions --shell bash --out-dir /tmp
# Source changes
gix <TAB>

---

I assume the version and CHANGELOG are automatically updated. Also, I am unsure if this should be under feat or BREAKING! - an exhaustive public enum has been changed, but I am not sure how/if it is used as a library.

[Skgland]

I don't think this command should assume that the user wants to install the completions system-wide, especially as that requires elevated permission (sudo). Most tools with an option to generate completions that I know output to stdout. This way the user can decide where to place the completions and no elevated permissions are required. I think a few of the tools I know by default add their completions by adding to bashrc and sourcing the output without ever writing to a file. Though that has other implications as the who knows what is sourced each time bashrc is run.

[ShaunSHamilton]

I don't think this command should assume that the user wants to install the completions system-wide, especially as that requires elevated permission (sudo).

Fair enough. I just thought this the simplest workflow (requires the least action from the user).

Most tools with an option to generate completions that I know output to stdout.

I have never had to do this for any tools. Do you have an example of one I could look at?

[Skgland]

One example would be rustup e.g. rustup completions bash

The help text when running rustup completions without specifying a shell include:
rustup completions bash cargo >> ~/.local/share/bash-completion/completions/cargo for how to setup the completions.
Though I think that is typically also done automatically as part of the rustup installation.

[Byron]

Thanks! This contributions comes at a surprise but I think it can be made to work.

You could make it so it's very similar if not the same as the same command for atuin.

In general, please do not attempt to auto-install these completions, being able to generate them is a good first step that already helps those who really want it.

@Skgland Please feel free to review as well, I think we want the same.

[Skgland]

I think the latest changes look good 👍

[Byron]

Thanks a lot!