feat(nushell): add nushell completion generation

[remmycat]

As mentioned on Discord, I noticed that clap offers completion generation for nushell in a separate crate. I went ahead and implemented these for the existing gen-completions command.

Some extensive implementation notes:

• When installing the new dependency, I also updated the clap and clap_complete versions in the manifests. I thought it couldn't hurt to have the whole group up to date when installing a new part. Looking at the lock file changes, I believe it had these versions already installed anyway.
• I moved the gen-completions code into a new module, similar to the other commands. With the extra impl it took up quite a bit of space in mod.rs otherwise.
• After trying out a few variants of composing the two crates into one command, I landed on defining an enum ourselves and adding some repetitive glue code. It was the most ergonomic version I could come up with, but I'm happy for alternative suggestions
• Question: I'm not sure if the nushell generations are wanted in the release tarball? I just noticed it when searching for mentions of completions.
• Another bit of code for the completions that I wasn't sure about was in atuin.nix. But the function used there, installShellCompletion, seems to only support bash fish and zsh in this way, so I didn't touch it (also I have next to no experience with nix, so I wouldn't want to break anything).
• I tested the generated completions using the latest nushell version, 0.90.1, and everything worked well for me.
• I'm gonna have a look at preparing a corresponding docs update PR tomorrow, or latest on the weekend, at least to update the list of supported shells for completion generation.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[ellie]

Looks good! thank you!