[DOC] Publish coverage report on Codecov #121

Make the coverage results public. Codecov supports the R language and it widely used.
According to #117, coverage is 100% 💪🏿

Resources

• example from codecov: https://github.com/codecov/example-r
• https://github.com/r-lib/actions/blob/v2/examples/test-coverage.yaml
• https://www.mzes.uni-mannheim.de/socialsciencedatalab/article/r-package/#subsection5-2 (July 2020)
• https://walczak.org/2017/06/how-to-add-code-coverage-codecov-to-your-r-package/ (June 2017)

Why not using SonarCloud?

We use SonarCloud for the bpmn-visualization TypeScript library, so why using a new tool/services?
Today (2022-10-26), SonarQube doesn't support the R language. See https://docs.sonarqube.org/latest/analysis/languages/overview/