Add CI

[developerjamiu]

There is no .github/ directory. Nothing runs analyze, tests or formatting on a pull request.

This was felt directly while reviewing #21: every check was manual.

Proposed workflow:

• flutter analyze
• flutter test
• dart format --set-exit-if-changed .
• flutter build ios --no-codesign and flutter build apk --debug on the example, once chore: allow url to launch on Android #18 makes the Android build work again
• flutter pub publish --dry-run on tags

CI should land before the bug fixes, since it protects every change after it.

Found during the v0.4.0 codebase audit.