Adds a GitHub Actions workflow to automatically run CI checks for the skills CLI tool. Tests are run on both Ubuntu, Mac and Windows for the `stable` and `dev` Dart SDK channels. The workflow triggers on pushes and pull requests to the main branch that modify the `tool/` directory, and runs:
- `dart pub get`
- `dart analyze --fatal-infos`
- `dart format` (on the dev channel only)
- `dart test` - is currently omitted as the test suite needs to be refactored to work properly without relying on real API keys in the CI environment
https://github.com/flutter/flutter/issues/183124