Use drupal/core-composer-scaffold to manage project creation and updates

[AlexSkrypnyk]

Use https://github.com/drupal/core-composer-scaffold to manage project creation and updates.

This will also allow to support composer create-project

For this, we would need:

1. Convert this repo to the proper Composer package (drupal/core-composer-scaffold can work only with packages downloaded by Composer rather than remote packages). This conversion will require a new major 2.x branch of the DrevOps Scaffold.
2. Publish repo to Packagist
3. Provide a Generalizer script/plugin that would be converting composer.json back from the plugin structure.
4. Provide mappings for all files that would need to be managed by the drupal/core-composer-scaffold plugin.
5. Update the testing system to use PHPUnit instead of Bats for any workflow testing.

---

Note that this will allow to perform updates on consumer sites using composer update drevops/scaffold manually. The customised files will be updated as well as composer.json.

The automated updates will be handled in #1321