[DX][D8] Provide a single command to install & run Backdrop

This is a new feature that has been added to Drupal 8.6.x: - Issue: https://www.drupal.org/project/drupal/issues/2911319 - Change record: https://www.drupal.org/node/2969396 - Documentation: https://www.drupal.org/docs/8/install/drupal-8-quick-start-command

It seems to rely on SQLite though, which we have dropped support for

Description: A new command to install and run Drupal was added.

...

php core/scripts/drupal quick-start demo_umami
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Congratulations, you installed Drupal!
Username: admin
Password: PM_kRw1i1xtPGeoT
Drupal development server started: <http://127.0.0.1:8888>
This server is not meant for production use.
One time login url: <http://127.0.0.1:8888/user/reset/1/insert-random-ness/login>
Press Ctrl-C to quit the Drupal development server.

Optional: Run the following command for a list of available options that you may need to configure quick-start:

$ php core/scripts/drupal quick-start --help

This is really neat! Perhaps we can do something similar for Backdrop?