pub.dev score: license, dartdoc, analysis, tests

- Add MIT license
- Add dartdoc to all public APIs (UxKeyboard, BendBox, Bezier, Json)
- Add analysis_options.yaml with flutter_lints and public_member_api_docs
- Add repository, issue_tracker, topics to pubspec.yaml
- Expand package description
- Fix BendBox constructor (const, super.key)
- Remove unused import in keyboard.dart
- Replace boilerplate tests with UxKeyboard smoke tests
- Move ux dependency to example's dependencies (not dev_dependencies)

analysis_options.yaml

@@ -0,0 +1,5 @@
+include: package:flutter_lints/flutter.yaml
+
+linter:
+  rules:
+    public_member_api_docs: true