Support the "extends" keyword in component templates

Problem/Motivation

It would be nice to be able to use the extends keyword e.g., in variant templates that make minimal modifications to the default template. If we use include or embed instead of extends, the "caller" isn't able to override Twig blocks in the original template.

Steps to reproduce

Use the extends keyword in a component template.

Proposed resolution

Adjust the way CL Components renders components in the Drupal theme to avoid the error:

Twig\Error\SyntaxError: A template that extends another one cannot include content outside Twig blocks.

Remaining tasks

User interface changes

API changes

Data model changes