Add an option to use Drupal's Twig environment for SSR

Problem/Motivation

For projects adding custom Twig extensions to Drupal's Twig environment, it might be convenient to add an option to use that for SSR instead of a default environment with no added extensions.

NB if enabled, the extensions would need to be added to the twig.js environment to ensure client-side rendering handles any extensions used.

Proposed resolution

Attached patch adds an admin form with an option to use the Drupal Twig environment (default: false).