Consider adding 'drupal-library' support to composer.json installer-paths

[jrockowitz]

Pantheon's composer.json file is very similar to the one used by the drupal-composer project.

Support for the drupal-library installer-paths is missing.

"web/libraries/{$name}": ["type:drupal-library"],

Someone noticed this issue with the Webform module's composer integration

Issue #3016309: Use composer merge plugin when document root is /web

[stevector]

Thanks @jrockowitz! Yes, we should add this change to keep example-drops-8-composer close to the drupal-composer project.

Another difference in that section of composer.json is "drush/contrib/{$name}": ["type:drupal-drush"] vs. "drush/Commands/{$name}": ["type:drupal-drush"]. @greg-1-anderson do you have an opinion on that difference?

[alisonjo315]

Cleaning up old browser tabs; this issue has been in an open tab for... a long time? Anyway.

Happy to tweak drush path, too, if that's desired.