Default verb treated as a value.

This was seen on version 2.8.0 targeting .NET Core 3.1 / 5.

Minimum code sample:
```
void Main()
{
var parser = new Parser();
var result = parser.ParseArguments(new[] { "copy", @"A:\", @"C:\", });
result
.WithParsed(a =>
{
Console.WriteLine(a.SourcePath);
Console.WriteLine(a.DestinationPath);
});
}

[Verb("copy", isDefault: true)]
class CopyArguments
{
[Value(0, MetaName = "source", Required = true)]
public string? SourcePath { get; set; }

[Value(1, MetaName = "destination", Required = true)]
public string? DestinationPath { get; set; }
}
````

This prints "copy" for `SourcePath` and "A:\\" for `DestinationPath`.

At this point in development I only have one verb (more verbs are planned), and that verb is/will be the default one.

Start by reproducing the sample through Parser.ParseArguments with the [Verb] and [Value] attributes shown. Trace how the default verb is handled before positional values are assigned, then add or update a regression test proving SourcePath receives A:\ and DestinationPath receives C:\ when the default verb is omitted from the arguments.

Escrita pelo modelo de indexação a partir do texto da issue.