fix(clap_complete_nushell): order positional arguments by index

[nitsky]

In clap_complete_nushell, arguments are written in the order they are added to the Command, not according to the
index they are specified with. If required positional arguments with a lower index are added to the command after
optional positional arguments are added, nushell rejects the generated completion because nushell forbids required
arguments to come after optional arguments.

For example, if positional arguments are defined as:

• .arg(Arg::new("second").index(2)) (optional)
• .arg(Arg::new("first").index(1)) (required)

The current output would incorrectly place second? before first, which nushell rejects.

To fix this, I reordered the output so all non-positional arguments are written in their normal order, and then all
positional arguments are written, ordered by their index.

I'm not certain this is the best approach, I would appreciate feedback.

[epage]

Feel free to edit commits to have them how we should review/merge them

[epage]

Also, our preference is to add tests like these in the commit before the fix with them passing, showing the broken behavior. The fix will then update the tests, showing the new behavior.

[nitsky]

@epage I apologize for the oversight. I split the change into two commits, one that adds the test, and another that implements the feature and updates the snapshots.

[epage]

Thanks!

CI seems to still be failing

[nitsky]

I fixed CI, I didn't see the nushell test that requires an unstable feature.

[epage]

Thanks!