Move dev dependencies to template project to make contribution easier

At the moment, our development scripts copy project_template/composer.json into the project root, mainly to ensure that we can add dev dependencies without affecting code meant for production. That said...our dev dependencies are just drupal/core-dev -- which is required by drupal/recommended-project and drupal/legacy-project -- and weitzman/drupal-test-traits, which is commonly used and quite handy for writing automated tests of real sites.

I see no reason why these two dev dependencies should not be made into dev dependencies for sites built with Drupal CMS. It would make contribution a little easier, and allow some helpful refactoring of the DDEV scripts.