feat(graphql): Awesome SchemaPrinter with directives, filtering, an…

…d advanced formating.

These are few known issues:
* Schema description is not yet supported by `graphql-php` (webonyx/graphql-php#1027)
* "implements" for interfaces is not yet supported by Lighthouse (just for the reference webonyx/graphql-php#728)

src/Observer/Dispatcher.php

@@ -4,6 +4,8 @@
 
 use Closure;
 use SplObjectStorage;
+use function array_values;
+use function iterator_to_array;
 
 /**
  * @template TContext