add nushell completions

[MilesCranmer]

cherry picked from #1230

Also I added #[arg(value_enum, value_name = "SHELL")] which makes the help message print the available shells.

This also bumps the Cargo.lock

[Copilot]

Pull Request Overview

This PR adds Nushell completion support to juliaup by introducing a custom completion shell enum and generic completion generator. It cherry-picks changes from #1230 and enhances the help message to display available shell options.

• Adds Nushell as a supported shell for tab completions
• Replaces direct clap::Shell usage with a custom CompletionShell enum
• Implements a generic completion generator that handles both standard shells and Nushell

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/command_completions.rs	Implements generic completion generator with Nushell support
src/cli.rs	Adds CompletionShell enum and updates Completions command
Cargo.toml	Adds clap_complete_nushell dependency

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[MilesCranmer]

Actually I think there is a way to simplify this... One sec

[IanButterworth]

I guess new tests are also needed?

[MilesCranmer]

Ok the new refactor avoids the string conversion, it's much cleaner

[MilesCranmer]

There's no tests of the completions utility but i can add some

[MilesCranmer]

Just pushed some tests; everything should be ready now I think

[IanButterworth]

LGTM