Support passing arguments to async main

[Austinpayne]

Adds a new static main function to AsyncParsableCommand to allow users to manually pass in arguments. The new functions behaves identically to the version found in ParsableCommand (see: https://github.com/apple/swift-argument-parser/blob/main/Sources/ArgumentParser/Parsable%20Types/ParsableCommand.swift#L126). This functionality is useful for manually building an argument list, e.g. when building a busybox~like binary.

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]

Thanks, @Austinpayne! This is a great addition — when I saw it I thought, "don't we have that already?"

Would you mind adding a test that verifies that this will parse and run an async command correctly? Thank you!

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test