Mark `CommandConfiguration` as `Sendable`

[MaxDesiatov]

Lack of this marker protocol conformance causes warnings with strict concurrency checks, such as Static property 'configuration' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

Checklist

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

[natecook1000]

Thanks, @MaxDesiatov! Didn't catch this one in #582 since the static configuration doesn't participate in a command's Sendable conformance.

[natecook1000]

@swift-ci Please test

[MaxDesiatov]

Ready to merge?