HookStyleListener - Convey hook results. Refine type-hints.

Note: This is uncommon and discouraged for new hooks, but some hooks require returning values, e.g

```php
function hook_foo() {
  return ['my-data'];
}
```

This should fix compatibility with those.

Civi/Core/Event/HookStyleListener.php

@@ -30,10 +30,11 @@ class HookStyleListener {
   }
 
   public function __invoke(GenericHookEvent $e) {
-    return call_user_func_array($this->callback, $e->getHookValues());
+    $result = call_user_func_array($this->callback, $e->getHookValues());
+    $e->addReturnValues($result);
   }
 
-  public function __toString() {
+  public function __toString(): string {
     $name = EventPrinter::formatName($this->callback);
     return preg_replace('/\(\$?e?\)$/', '(&...)', $name);
   }