Allow classes to be wrapped inside a store

[timbasel]

Summary

This PR allows classes to be wrapped inside a store.
It implements the very small change outlined by @Reinhard2019 in the Issue #1661.

Even if the issue was closed as wontfix I do believe that the functionality is tremendously useful.
It has no effects on existing code because it is an opt in on a class by class bases.

To make a class wrappable you just define the $WRAP property (I've added a minimal helper function to do this) in the constructor of the class.

class Test {
  constructor() {
    makeWrappable(this);
  }
}
const [store, setStore] = createStore(new Test())

You can even make just individual instances wrap, by calling makeWrappable just on the created instance.

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[ryansolid]

Directionally I'm not opposed. I've considered adding symbol or having helpers to do the wrapping. Technically you can add the PROXY symbol to your class and define your own wrapper today.

My thinking though is like MobX there is perhaps more involved than just auto wrapping every property. I think we should consider something like MobX makeObservable if we go this way. And I think that requires a bit more thought than just enabling it.

[timbasel]

Just using $PROXY unfortunately did not work for me. I can't remember what exactly broke, but I can check if want.

I'm not really familiar with MobX and have just looked at the documentation for makeObservable, but from my perspective it just over-complicates the matter. For my use case I just want to treat a class instance the same as a basic object when creating a store from it.
In my project I have a few thousand of wrapped classes and I have not noticed any performance issues or differences.