Decouple decorators from reactivity infra of engine

[davidturissini]

Description

Today, decorators are tightly coupled to engine reactivity. We should de-couple these so decorators can be moved into their own repo and have limited knowledge of engine internals.

[caridy]

part of #1038, but there is more to do here. at first glance, if each engine version contains its own mechanism to observe objects passed into instances controlled by that engine version, it should be fine to have them coupled.

[caridy]

This will partially solved by #1417

[jmsjtu]

We plan to use signals for reactivity and native decorators in the future, closing this issue.