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/Foundation/ErrorServiceProvider.php

@@ -71,6 +71,6 @@ class ErrorServiceProvider extends AbstractServiceProvider
             );
         });
 
-        $this->app->singleton(Reporter::class, LogReporter::class);
+        $this->app->tag(LogReporter::class, Reporter::class);
     }
 }