Support grouping by multiple fields with @indexBy directive

[ruudk]

{
   customers {
        edges @indexBy(field: "node.id,node.name") {
            node {
                id
                name
            }
        }
    }
}

Generates the following code:

/**
 * @var array<int,array<string,CustomerEdge>>
 */
public array $edges {
    get => $this->edges ??= (function() {
        $result = [];
        foreach ($this->data['edges'] as $item) {
            $result[$item['node']['id']][$item['node']['name']] = new CustomerEdge($item);
        }
        return $result;
    })();
}