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/Locale.php

@@ -16,7 +16,7 @@ use Flarum\Locale\LocaleManager;
 use Illuminate\Contracts\Container\Container;
 use RuntimeException;
 
-class Locale
+class Locale implements Extender
 {
     protected $directory;