Support passing a negative number as an argument

Imagine a command like this

```swift
import ArgumentParser

struct Absolute: ParsableCommand {

@Argument() var number: Int

func run() throws {
print(abs(number))
}
}

Absolute.main()
```

you can run

```
command 5
```

and it works. but running

```
command -5
```

fails with `Error: Missing expected argument ''` which makes sense.

I would have expected being able to pass the argument as `"-5"` but that doesn't work either.

Any advice on how to handle this?

Start by reproducing the Absolute.main example with @Argument() var number: Int, comparing command 5 with command -5 and the quoted form. Trace how positional Int arguments handle option-like tokens; done means negative values are accepted while normal option parsing and existing argument behavior remain intact.

Written by the indexing model from the issue text.