Enable scoverage in github actions and add coveralls plus badge

[mdedetrich]

This PR does the following things

1. Enables scoverage in github actions so it actually runs on pull requests
2. Adds coveralls (which works in conjunction with scoverage)
3. Adds a badge which shows the status of the coverage of the project. Note that it takes roughly a day for coveralls to start displaying the coverage for a new project

[mdedetrich]

Hmm, so it seems that this is not working because scoverage doesn't support Scala3 yet. Theoritically its possible to put conditionals so that scoverage/scoveralls is only run for Scala2 but since this is non urgent I think just waiting for Scala3 support in scoverage is better.

[mdedetrich]

Upstream issue at https://github.com/scoverage/scalac-scoverage-plugin/issues/299