Support `pub` package manager for dart and flutter

[pub.dev](https://pub.dev) essentially uses Semver with minor deviations, which they have explained in detail here:

- https://dart.dev/tools/pub/versioning#semantic-versions
- https://github.com/dart-lang/pub_semver/blob/master/README.md#semantics

They have implemented [pub_semver](https://github.com/dart-lang/pub_semver), but unfortunately, it is only available in Dart.