The repository already contains the foundation for PHP unit testing and linting, particularly the latter already works out of the box right now. This issue is about completing the setup so that the repository offers easy PHP unit testing capabilities that can be run locally and in CI (GitHub actions) without separately setting up WordPress or a MySQL database.

The suggested approach is to mostly copy how Gutenberg does this, as it allows us to rely on an established approach and reuse the @wordpress/env package - this way we don't need to worry about most of the complexities, plus we get a development environment for the plugin out of the box.

For the scope of this issue, linting and testing of non-PHP code (e.g. JS, CSS) should be excluded. This is to keep things simple and since PHP linting and test coverage is the most crucial for this plugin initially, since the plugin at its foundation is PHP, and we don't have any JS or CSS code yet.