feat: implement shell completion helpers

[wolfv]

This is a bit of a starting point to add support for sourcing shell completions in rattler shell.

[ruben-arts]

Let's merge this while we're working on the pixi implementation, this is untested code that can easily be extended with more features. So let's do it when there is something/one that uses it.

[wolfv]

We need to decide on the API here. We now have a high-level command that is able to source completion files from a given directory.

I think the question is whether the input to the API should be a the final directory (ie. prefix.join(completion_dir) or whether it should be just the prefix, and rattler will stick things together.

The benefit of giving the final dir directly would be that for pixi global we could arrange folders a bit different (e.g. .pixi/completions/bash, .pixi/completions/fish, ...).

Otherwise we would be forced to do: .pixi/completions/share/bash/bash-completions/.... If we give the directory directly, the calling code for pixi shell should look something like:

if let Some(completions_dir) = shell.completions_dir() {
    shell_script.source_completions(prefix.join(completions_dir));
}

WDYT @ruben-arts / @Hofer-Julian ?

[wolfv]

I decided to do the following API:

shell has an completions_dir() -> Option<PathBuf> that returns the default completion folder location.

The sourcing of completions requires to give the full folder to the folder with completions then though. So we can put them into a different location for pixi global.