Introduced the Core Project Message composer plugin

A new Composer plugin, the Core Project Message plugin, was introduced and added to both the Recommended Project and the Legacy Project templates.

Plugin documentation page here: https://www.drupal.org/docs/develop/using-composer/using-drupals-project...

The Core Project Message plugin's only purpose is to print a message at the end of a composer create-project or a composer install command. The plugin gives the capability for a project to provide the follow-on message either via extra data in the project's composer.json file, or in a separate text file.

By default, the Core Project Message plugin is configured to tell the users what they should do next after running composer create-project (including instructions on how to remove the Core Project Message plugin). It is also configured to warn users of the drupal/drupal project that the git clone workflow is only intended for Core development, and production sites should not be installed this way.