[postponed] Use Twig as the default theme engine

After the conversion to twig, Drupal will need to be updated to use the twig engine by default.

This issue is postponed until #1757550: [meta] Convert core theme functions and templates to Twig is resolved.

We will also need to un-do the ability for a single theme to use more than one engine, reverse patch is available here:
#1938948: Temporarily allow PHPTemplate themes to use .html.twig templates during Twig conversion (comment-7161890)