Support multiple error reporters

The error handling middleware now expects an array of reporters.
Extensions can register new reporters in the container like this:

    use Flarum\Foundation\ErrorHandling\Reporter;

    $container->tag(NewReporter::class, Reporter::class);

Note that this is just an implementation detail and will be hidden
behind an extender.

src/Api/ApiServiceProvider.php

@@ -53,7 +53,7 @@ class ApiServiceProvider extends AbstractServiceProvider
             $pipe->pipe(new HttpMiddleware\HandleErrors(
                 $app->make(Registry::class),
                 $app->make(JsonApiRenderer::class),
-                $app->make(Reporter::class)
+                $app->tagged(Reporter::class)
             ));
 
             $pipe->pipe($app->make(HttpMiddleware\ParseJsonBody::class));