Base all contribution and development on DDEV

I think that, for ease of maintenance and simple consistency, there is no purpose in supporting development of Drupal CMS on any platform other than DDEV. Note that this isn't about how consumers of Drupal CMS can build their own projects -- it's about how we'll develop Drupal CMS itself.

Since we have to basically redo the infrastructure anyway in our move to GitLab, let's take this opportunity to add DDEV configuration for this project. Contributors, once they clone the repo, will be able to spin it up with this:

ddev start
ddev drupal-install
ddev launch

Doesn't get a lot nicer than that.

I'll follow this with other MRs that will deal with how actual projects are spun up -- those do need to support both DDEV-based development and bare metal.