[whatwg/streams] Symbol.clone to ease-out structuredClone implicit conversion (Issue #1314)

### What problem are you trying to solve?

The complexity of cross-realms interactions is very high these days:

  * *Workers* and `postMessage` implicitly use *structuredClone* to pass values around
  * *Proxy* used in foreign worlds can't survive any `postMessage` dance without explicit user interaction (i.e. use ad-hoc API to transform that proxy into something consumable elsewhere)
  * classes defined in a world cannot be cloned in any other world, including the very same realm they'd like to be cloned

### What solutions exist today?

None, but we historically have `toJSON` convention to automatically transform any class into its *JSON* representation. Times are different these days though, and while `structuredClone` works wonderfully for all its allowed types, it's impossible from a library author point of view to grant some data can cross boundaries or be cloned properly, as opposite of throwing errors.


### How would you solve it?

```js
const ref = {
  // implicitly invoked when `structuredClone(ref)` happens
  // or during the conversion / clone algorithm is applied
  [Symbol.clone]() {
    // returns any structured-clone compatible value
    return new Map(Object.entries(this));
  }
};
```

The algorithm should look for non primitive values to a `Symbol.clone` special property that should return the cloned representation of the underlying proxy, class, complex data, and so on.

For this proposal, it would be a developers concern to "*reconstruct*" or understand returned data, simply screening in a recursive way whatever was cloned, simplifying ad-hoc clones for API calls, or cross-proxy related use cases.


### Anything else?

As library author, I could dictate how any reference the library creates could be cloned or even throw if some reference should actually never be cloned (authentication / credentials / passwords / security related things / ... and so on).

This was (wrongly) posted also in *TC39* in case some extra thoughts are needed: https://es.discourse.group/t/symbol-clone-to-ease-out-structuredclone-implicit-conversion/2035

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