Re-introduce Compat extender

Turns out Container::call() does not work with invokable classes.
Thus, we need to wrap callables in a custom extender class to
support injecting any resolvable type-hint automatically.

Refs #851.

src/Extend/FormatterConfiguration.php

@@ -15,7 +15,7 @@ use Flarum\Formatter\Event\Configuring;
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Events\Dispatcher;
 
-class FormatterConfiguration
+class FormatterConfiguration implements Extender
 {
     protected $callback;