Allow creation of the model without decorators

### Expected Behaviour

The library should allow for the creation of the model without the need of decorators.
Something similar to what [Mobx](https://github.com/mobxjs/mobx) is having would be a nice enhancement.
https://mobx.js.org/best/decorators.html

The issue does not name any files, tests, or entry points. Start by reading how the repository currently creates models with decorators, then compare that flow with MobX's decorator-free approach. Done should mean models can be created without decorators, with coverage for the new API and existing behavior preserved.

Written by the indexing model from the issue text.