Add test coverage collection with coveralls

[jonasfj]

See example setup in:
https://github.com/dart-lang/yaml_edit/blob/main/.github/workflows/test-package.yml

[jonasfj]

Fixed with #165

We can do more coverage collection from integration tests (android, ios), if we want some day in the future. But I'm not absolutely no rush to do that. It'll make our infrastructure much more fragile, so we'll spend more time debugging weird test failures.

I think we have a better off trying to increase coverage by writing more tests.