Re: [whatwg/dom] Allow postMessage-ing AbortSignal (#948)

With the new `abortSignal.reason` synchronous observation doesn't really make sense anymore as one also needs to send the reason across the thread which is considerably less simple than the setting of a boolean cross-thread, however the asynchronous part of the API still would make sense.

For this we could have a design more like:

```txt
AbortSignal serialization steps given _abortSignal_ and _serialized_:

1. If _abortSignal_ is **aborted**.
  a. Set _serialized_.[[AbortReason]] to _abortSignal_'s **abort reason**.
  b. Set _serialized_.[[MessagePort]] to **undefined**.
2. Else.
  a. Let _port1_ be a new MessagePort in the current realm.
  b. Let _port2_ be a new MessagePort in the current realm.
  c. Entangle _port1_ and _port2_.
  d. Set _serialized_.[[AbortReason]] to **undefined**.
  e. Set _serialized_.[[MessagePort]] to the **sub-serialization** of **port2**.
  f. Add the following **abort algorithm** to _abortSignal_:
    a. Post a message consisting of _abortSignal_'s **abort reason** to _port1_.
    b. Disentangle _port1_.
    
AbortSignal deserialization steps given _dataHolder_ and _value_:

1. If _dataHolder_.[[AbortReason]] is not **undefined**:
  a. Abort _value_ with _dataHolder_.[[AbortReason]].
2. Else.
  a. Assert _serialized_.[[MessagePort]] is a MessagePort.
  b. Add a handler for _serialized_.[[MessagePort]]'s "message" event with the following steps:
    i. **abort** _value_ with the *data* of the message.
    ii. Disentangle _serialized_.[[MessagePort]]
```

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