Fix issue with unconditional remaining properties in subcommands

[natecook1000]

This extends the current behavior of @Argument array properties defined with the .unconditionalRemaining parsing strategy to work with subcommands, as well. For example, with a command hierarchy like this:

@main
struct Root: ParsableCommand {
    static var configuration: CommandConfiguration {
        .init(subcommands: [Sub.self])
    }

    @Flag var verbose = false
}

struct Sub: ParsableCommand {
    @Argument(parsing: .unconditionalRemaining)
    var remainder: [String] = []

    @OptionGroup var root: Root

    func run() {
        print("Verbose:", root.verbose)
        print("Remainder:", remainder)
    }
}

We want the Sub command to capture any arguments after the subcommand name appears in the command-line arguments. This fix enables the following behavior:

$ root --verbose sub foo --bar
Verbose: true
Remainder: foo --bar
$ root sub foo --bar --verbose 
Verbose: false
Remainder: foo --bar --verbose
$ root --verbose sub foo --bar --verbose 
Verbose: true
Remainder: foo --bar --verbose

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