Implement localdev setup and DrupalCI test harness

Problem/Motivation

The module currently has a basic Behat feature in the example/ folder, along with an example module to enable some test fixtures. However, there is no way within the module to run that test, and nothing automated. For efficient maintenance, it's very helpful to have an easily replicable localdev environment, and to have even a minimal test suite that runs automatically in DrupalCI.

I plan to refactor the module structure, and introduce a DDEV configuration and setup to install a test site and run tests against it, as well as add a .gitlab-ci.yml config and enable DrupalCI pipelines for the project.

Remaining tasks

• Add localdev DDEV environment into the project
• Create .gitlab-ci.yml and get Pipeline working
• Implement minimal test suite to demonstrate CI works and basic functionality for the module.