Unexpected “Asynchronous subcommand of a synchronous root”

When attempting to create an application with a AsyncParsableCommand root and AsyncParsableCommand subcommands, a fatal error is thrown at runtime “Asynchronous subcommand of a synchronous root.”

**ArgumentParser version:** `1.2.0`
**Swift version:** `swift-driver version: 1.62.8 Apple Swift version 5.7 (swiftlang-5.7.0.127.4 clang-1400.0.29.50)
Target: arm64-apple-macosx13.0`

### Checklist
- [ ] If possible, I've reproduced the issue using the `main` branch of this package
- [x] I've searched for [existing GitHub issues](https://github.com/apple/swift-argument-parser/issues)

### Steps to Reproduce

source:

```
import protocol ArgumentParser.AsyncParsableCommand
import struct ArgumentParser.CommandConfiguration

@main
struct asyncissue: AsyncParsableCommand {
static var configuration = CommandConfiguration(subcommands: [Cmd1.self])

struct Cmd1: AsyncParsableCommand {
mutating func run() async throws {
print("Hello")
}
}
}
```

### Expected behavior

When resulting command is run, help with a subcommand would be displayed.

### Actual behavior

```
ArgumentParser/ParsableCommand.swift:184: Fatal error:
--------------------------------------------------------------------
Asynchronous subcommand of a synchronous root.

The asynchronous command `Cmd1` is declared as a subcommand of the
synchronous root command `asyncissue`.

With this configuration, your asynchronous `run()` method will not be
called. To fix this issue, change `asyncissue`'s `ParsableCommand`
conformance to `AsyncParsableCommand`.
--------------------------------------------------------------------

zsh: trace trap swift run
```

Reproduce the sample application with Swift ArgumentParser 1.2.0, then inspect ParsableCommand.swift around line 184 where the fatal error is reported. Verify the root and subcommand conformances used by the sample; done means the asynchronous root and subcommand run without the synchronous-root fatal error and display help as expected.

Written by the indexing model from the issue text.