Object based data models

[platosha]

Discussed in https://github.com/vaadin/hilla/discussions/1634

Following the aforementioned RFC and the discussion, let us replace the class-based models with the object based design.

This is a big change. I would suggest to break it down onto steps, roughly:

• [ ] Add a separate package packages/ts/model with the model primitives and builder APIs.
• [ ] Add a generator plugin to emit models in the object-based format, which would initially coexisting with the class-based models.
• [ ] Update the form and react-form libraries to use the object-based models.
• [ ] Update the CRUD helpers to use the object-based models.
• [ ] Remove the class-based models generator.
• [ ] Update documentation and example apps.