SA1208 - Missing file-scoped namespace support

SA1208 SystemUsingDirectivesMustBePlacedBeforeOtherUsingDirectives is not fired when a file-scoped namespace is used.

Example:
```cs
namespace WebApi.Controllers;

using Microsoft.AspNetCore.Mvc;
using System;

[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
}
```

Environment:
- .NET 6
- C# 10
- StyleCop 1.2.0-beta.376