Add Valet 4 version

Valet 4 requires some [changes to the Drivers synthax](https://laravel-news.com/valet-v4-is-released#content-custom-drivers).

1. Adding namespace
```
namespace Valet\Drivers\Custom;
use Valet\Drivers\ValetDriver;
```
1. Add attribute typing
```
public function serves(string $sitePath, string $siteName, string $uri): bool
...
public function isStaticFile(string $sitePath, string $siteName, string $uri)/*: string|false */
...
public function frontControllerPath(string $sitePath, string $siteName, string $uri): string
```

Start by locating the custom Valet driver implementation and compare its methods with the Valet 4 custom-driver syntax in the linked Laravel News article. The work is done when the driver uses the requested namespace and compatible method attribute types.

Written by the indexing model from the issue text.