ci: use GitLab CI/CD & remove Github Actions

Fixes #2 (closed).

Removes both the normal workflows from Github Actions, as well as deleting the Dependabot configuration. We'll need to bring the latter back at a later point in time.

Creates a pipeline definition which runs for all branches, either in a branch or merge request pipeline depending on whether merge requests exist for a given branch.

Alters the versions used for Node.js to the LTS versions 12, 14 & 16.

Creates parallel pipelines for each of the versions of Node.js.

Runs one, shared linting job, only on merge requests and non-protected branches. The idea behind that is that you never work on a protected branch, thus linting is not necessary to run outside of merge requests. You might say the same of tests, but seeing as protected branches should contain working code, it is far more important that it works than whether or not pre-existing code does not conform to the style guidelines used.

Caveat: with the test reports & coverage included this is not an exact copy of the Github Actions workflow. It's easy to split out if that's preferred: give me a @-ping, and I'll remove it.