Turn extenders into callables

This simplifies the API and gives extension developers more
flexibility, for a) maintaining backwards compatibility, and
b) doing advanced stuff that extenders do not allow.

Note that only extenders are guaranteed to work across
different versions of Flarum (once the API surface is stable).

See the discussion in https://github.com/flarum/core/pull/1335.

src/Extension/ExtensionServiceProvider.php

@@ -24,11 +24,11 @@ class ExtensionServiceProvider extends AbstractServiceProvider
         $this->app->bind('flarum.extensions', ExtensionManager::class);
 
         $this->app->booting(function (Container $app) {
-            /** @var \Flarum\Extend\Extender[] $extenders */
+            /** @var callable[] $extenders */
             $extenders = $app->make('flarum.extensions')->getActiveExtenders();
 
             foreach ($extenders as $extender) {
-                $extender->apply($app);
+                $app->call($extender);
             }
         });
     }