Add code coverage reporting
Follow-up from #49 (closed)
Let's add test coverage reporting to the pipeline. We have this working on the Nanoc site for rspec tests (https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab/ci/test.gitlab-ci.yml?ref_type=heads#L117), and it'd be helpful here, too.
Let's see if we can get this going for Go tests and Jest tests.
Docs: https://docs.gitlab.com/ee/ci/testing/test_coverage_visualization.html