Simplify CommonMark extension registration

From slack convo with colin: https://drupal.slack.com/archives/C012JG7EWKU/p1589468887143700

- Extend \Drupal\markdown\Plugin\Markdown\CommonMark\ExtensionInterface from \League\CommonMark\Extension\ExtensionInterface.
- Remove internal render/parser interface detections (let extensions do that themselves)