feat: Add automated license scanning

[eseidel]

I've done license scanning of our Dart dependencies a couple times with: https://pub.dev/packages/dart_license_checker

There appear to be other similar tools, e.g. https://pub.dev/packages/license_checker and https://verygood.ventures/blog/very-good-cli-license-checker

Would like our CI to fail if any package depends on a non-allowed license.