code: public function extract(string $namespace): Config
      {
          $extracted = $this->get($namespace);
          if (!is_array($extracted)) {
              throw ConfigException::forExtractionFailure($namespace);
          }
  
          return new self($extracted);
      }
docs: Returns new instance of the config containing only values from the
  given namespace.
  
  @param string $namespace
  @return Config