Handle Swift 6.2 sendability changes

[natecook1000]

Adds SendableMetatype conformance to the ExpressibleByArgument and ParsableArguments protocols, enabling existential use in Swift 6.2.

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

[natecook1000]

FWIW the API breakage check only passes because it's using a 6.0 compiler and not a 6.2 compiler that would see the SendableMetatype requirement. That said, I don't believe this will be source breaking in practice because:

• it will only affect those using the Swift 6.2 compiler or greater
• it will only pose a problem for types with isolated conformances to ParsableArguments or ExpressibleByArgument, which there shouldn't really be any yet

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[natecook1000]

With the updated approach the API breakage check fails now, but the notes above still apply.