Introduce test coverage with codecov

[lbm364dl]

Code coverage is another good metric for trusted code. Most R packages seem to integrate with https://about.codecov.io/. Read about this. I think it's free for open source repositories and our usage.

[lbm364dl]

Overall seems to be working but it says the GitHub app wasn't installed correctly (#55 (comment)). We should check that when @eduaguilera is back. Also there seems to be another GitHub app from them that allows to use AI to suggest test generation and it seems to be free for now. It would also be worth trying that (https://docs.codecov.com/docs/beta-codecov-ai).

Reopening until then.