Don't load DuskServiceProvider if dusk doesn't exist

When running tests without dusk install this triggers an error

src/TwillServiceProvider.php

@@ -175,7 +175,7 @@ private function registerProviders(): void
             $this->app->register($provider);
         }
 
-        if (app()->environment('testing')) {
+        if (app()->environment('testing') && class_exists(Laravel\Dusk\Browser::class)) {
             $this->app->register(DuskServiceProvider::class);
         }