Introduce Drupal Scaffold Composer plugin

This change is part of the Composer initiative: #2958021: Proposal: Composer Support in Core initiative

We've added the drupal/core-composer-scaffold package to core, AKA 'the scaffold plugin.'

It's a Composer plugin that allows the root package to define a docroot for a Drupal site to be different from the root directory of the project. Restated: You can use it to move the vendor directory out of the docroot of the HTTP server.

This package allows Drupal core to build out its own docroot filesystem, similar to the drupal-composer/drupal-scaffold package available from third parties.

See the documentation here: https://www.drupal.org/docs/develop/using-composer/using-drupals-compose...

This change was a blocker for #2982680: Add composer-ready project templates to Drupal core which will introduce a ‘legacy’ project package for installing Drupal core with the vendor directory in the docroot, much like the tarball does now.