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.

先使用所示的 [Verb] 和 [Value] 屬性，透過 Parser.ParseArguments 重現範例。追蹤預設 verb 在指派位置值之前的處理方式，然後新增或更新回歸測試，證明從引數中省略預設 verb 時，SourcePath 會接收 A:\\，DestinationPath 會接收 C:\\。

由索引模型根據 Issue 內容生成。