Feature Request: Polymorphic Reference Folding

It'd be cool to be able to both load and fold on Polymorphic references. Something like...

```js
const author = await em.findOne(
Author,
{},
{
populate: {
// Poly on Book | Blogpost | Article
latest: {
_book: ["name", "chapters", "store"],
_blogpost: ["siteName", "link"],
_article: ["publication", "url"],
},
},
},
);

const currentBlurb = author.latest.fold({
Book: (b) => `${b.name} now available at ${b.store.get.name}`, // `b` is Loaded
Blogpost: (bp) => `See the latest from ${author.name} at ${bp.siteName} (${bp.link})`,
Article: (a) => `${a.publication.get.name} recently wrote about ${author.name}. See what they had to say! (${a.link})`,
});
```

[This](https://stackoverflow.com/a/64528509/12022691) TypeScript type might be helpful.

It might be nice to avoid `.get` if possible, to save a few chars. Not sure how that might play out in Arrays, i.e. if "Reviews" was a poly array on `UserReview | PublicationReview` would the preferred syntax be `Book.reviews.get.map((poly) => poly.fold({ ... }))`?

I think this would scale pretty well to Async Properties, derived fields, maybe some Persisted stuff, Hooks, etc.