Add test coverage

[nickmeinhold]

• [x] get coverage in CI
• [x] add a badge with coverage to README
• [x] send to https://coveralls.io/ ?
  • hell yes! there is a github action that includes changes to test coverage in a PR... amazing.
  • Coveralls GitHub Action · Actions · GitHub Marketplace

We could use this: pulyaevskiy/test-coverage: Runs tests in Dart VM and collects coverage data.

Not sure if we need the above package? Can we just use flutter test --coverage?