SA1400 false positive for new `extension` keyword

False positive SA1400: Access modifier should be declared

```cs
internal static class ExceptionPolyfills
{
    extension(Exception) // SA1400: Element '.ctor' should declare an access modifier
    {
    }
}
```

See [C# 14 – Exploring extension members](https://devblogs.microsoft.com/dotnet/csharp-exploring-extension-members/)