Remove Never return from ParsableCommand.main()

[natecook1000]

Description

This brings the main() method in line with using the new @main attribute on the root
command type.

Detailed Design

The changes here are to make ParsableCommand.main() a Void-returning function, to break out a separate no-argument version, and to remove the default parameter. The default-argument version doesn't satisfy the () -> Void requirement for @main.

The main() method now returns without exiting on successful execution of a

Documentation Plan

No real impact. Documentation for using @main with the library to follow.

Test Plan

All existing tests continue to pass. SwiftPM will need an update to support @main in executable targets.

Source Impact

Typical code should continue to behave as expected, but this is source-breaking if anyone has a strongly-typed usage of ParsableCommand.main(), e.g. let mainFunction: ([String]?) -> Never = MyCommand.main. Keeping the old version with a deprecation/unavailability annotation isn't possible, since there would be ambiguity between that and the new ones.

In addition, code that followed MyCommand.main() in main.swift is no longer dead code, and will be executed after successful execution of the command.

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]

@swift-ci Please test