Setup GitLab CI pipelines for phpcs, phpstan, eslint, composer validations

MR created.

Hmm, tests are failing because the module has no support for D11 yet. Let's fix that.

Well, now tests are not failing because of D11 support anymore. However there are multiple confusing errors when running even just phpstan locally. It seems the tests are fundamentally incompatible with D11. Next steps perhaps: look into how modules have kept supporting D10 and D11 with the test base class changes. (Or prove me wrong in my analysis.)