Test coverage changes tracking

## Background [Optional]
I'm wondering if it would be possible to track test coverage _changes_?

The current test coverage level may be low, but we can eventually improve if new PRs are not decreasing the number.

Sorry this issue exists. But it is solely because I didn't notice that scoverage does not track test coverage level for Scala 2.13, and I let the coverage level decrease quite a bit.

## Question
I'm not sure if this can me achieved with the reasonable amount of effort. But if we can add a GitHub action check that fails if a new PR decreases test coverage level, then it would encourage gradually adding test coverage to areas not very well covered now.