public function addFolder($folder, $extension): ConfigCollection {
        $extLen = strlen($extension);
        $files = scandir($folder, SCANDIR_SORT_ASCENDING);
        foreach ($files as $file) {
            if (in_array($file, ['.','..'])) {
                continue;
            }
            if (substr($file, -$extLen) != $extension) {
                continue;
            }
            $config = FileConfig::load(paths($folder, $file), false);
            if ($config) {
                $this->addConfig($config);
            }
        }
        return $this;
    }