Adds a new `AsyncParsableCommand` protocol, which provides a
`static func main() async` entry point and can call through to the root
command's or a subcommand's asynchronous `run()` method. For this
asynchronous execution, the root command must conform to `AsyncParsableCommand`,
but its subcommands can be a mix of asynchronous and synchronous commands.
Due to an issue in Swift 5.5, you can only use `@main` on an
`AsyncParsableCommand` root command starting in Swift 5.6.
This change also includes a workaround for clients that are using Swift 5.5.
Declare a separate type that conforms to `AsyncMainProtocol` and add the `@main`
attribute to that type.
```
@main enum Main: AsyncMain {
typealias Command = <#command#>
}
```
Nate Cook