WIP: extensibility draft

[kosich]

This allows adding functions on the proxy fields (fix #3)

E.g.:

interface Extension {
  readValue<S>(this:S): S extends ObservableProxy<infer A, unknown> ? A : never;
}

const x: Extension = {
  readValue() { return this… }
}

const source = proxify(of({ a: 1 }, { a: 2 }), x);
const d = source.a.readValue() // d: number

This might be handy if combined with rxjs-autorun:

// RxJS :: Autorun + Proxify 
const s = statify({ a: '🐰', b: '🏡' });
autorun(() => s.b.$() + s.a._()); //🏡🐰
s.b.next('🛸'); //🛸🐰

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I don't quite like the typing output and code should be refactored to include this feature.
Also, it's not clear at what level should this extension be applied and when dropped (observable/subject/behaviorSubject)