Support for setting a Promise as handled to true

[joanlopez]

On certain scenarios, like for instance when adding support for Streams API (more specifically to ReadableStreamReaderGenericInitialize), a mechanism to mark a Promise as handled is required, as per spec's requirements:

(from the link above)

...
    // 2. Set reader.[[closedPromise]] to a promise rejected with stream.[[storedError]].
    // 3. Set reader.[[closedPromise]].[[PromiseIsHandled]] to true.

That method I guess would not only set Promise.handled to true, but also do any other operation related with the handling of the Promise.

In Deno, for instance, there's a setPromiseIsHandledToTrue function, used specifically for that.

Alternatively, would be nice to have a more generic set of handlers for Promises management, which would also be handful for this specific use case.

Thanks!