This adds underscored initializers that let library users add `= nil` to
declarations of optional `@Option` and `@Argument` properties. Previously,
default values have been available for properties of non-optional types
only.
These new initializers use `_OptionalNilComparisonType` as the wrapped
value parameter, so only a `nil` literal is acceptable in the default
value position. This avoids the problem of declaring an optional property
with a non-`nil` default, which ends up negating the purpose of an optional.
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