[CodeQuality] Add EventSubscriberMethodReturnVoidRector

[TomasVotruba]

Add new code quality rule EventSubscriberMethodReturnVoidRector.

Event subscriber methods that hook into an event must return void — the event is a class passed by reference, so returning it has no effect.

How it works

1. Class implements EventSubscriberInterface and has getSubscribedEvents().
2. Parses the getSubscribedEvents() return array to collect hook method names. Supports all formats:
   • 'event' => 'method'
   • 'event' => ['method', $priority]
   • 'event' => [['m1', 10], ['m2']]
3. For each registered hook method that returns the event, sets return type to void and rewrites return $event->setX(...) into $event->setX(...) (keeping an early return; inside branches, dropping it on the final statement).
4. Skips methods already void or returning a non-event value. Non-registered methods are left untouched.

Before

public function onEvent(SomeEvent $event): SomeEvent
{
    if (...) {
        return $event->setResult($response);
    }

    return $event->setFailed(...);
}

After

public function onEvent(SomeEvent $event): void
{
    if (...) {
        $event->setResult($response);
        return;
    }

    $event->setFailed(...);
}

Registered in the symfony-code-quality set.