Support async custom completion closures

[rgoldberg]

Support async custom completion closures.

Resolve #555

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rgoldberg]

@natecook1000 This PR should be simpler to review than #764, so you might want to get this out of the way first.

Thanks for reviewing / merging everything.

[natecook1000]

Looks good! Can you add a couple tests that verify that the async version is usable?

[rgoldberg]

@natecook1000 Added a multi-shell custom completion test for async custom completion handlers.

Thanks for reviewing.

[natecook1000]

LGTM!

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[rgoldberg]

@natecook1000 I pushed an attempt to fix the Linux build issues. I didn't get any build or test issues on macOS 15.5, but I don't know how to run the Linux tests myself, or how to make the macOS tests mimic the Linux tests.

I assume that the Linux tests have stricter concurrency checking enabled than the macOS & Windows tests do. Maybe they should be changed to be as strict as the Linux tests. Should I open an issue for that? Does one already exist?

[rgoldberg]

@natecook1000 Is there any way to get this into 1.5.1? I fixed the issue with the Linux tests, so it should be good to go. Thanks.

[rgoldberg]

@natecook1000 Sorry, I just noticed that 1.5.1 only includes the small patch, not all the changes for completions, etc.

I was so worried that these 2 PRs wouldn't get in the new release that I hurried to rebase & comment before completely understanding what was in 1.5.1.

I also wondered why it was 1.5.1 instead of 1.6.0, but now I know. Sorry for the misunderstanding. Looking over the unreleased notes now.

[natecook1000]

@swift-ci Please test