worker: populate BroadcastChannel MessageEvent source

[SudhansuBandha]

Populate the MessageEvent source property with the sender's worker thread ID for BroadcastChannel messages originating from worker threads.

Fixes: #59053

[jdmarshall]

In the MDN documentation, https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent/source

The source read-only property of the MessageEvent interface is a MessageEventSource (which can be a WindowProxy, MessagePort, or ServiceWorker object) representing the message emitter.

The threadId might be sufficient to work with the new API you folks were working on but I'm wondering if there's a technical limitation that makes diverging from the web api an absolute necessity? Ideally this would be the Worker Object, since one could immediately invoke postMessage() for any rendezvous coordination necessitated by the broadcast. With that in mind a MessagePort would also work if that's available, but I'm guessing that is not...

[jasnell]

This is a breaking change making this a semver-major. It will need doc updates as well. I think we should instead try to find a way of doing this so that it's not a breaking change

[SudhansuBandha]

Thanks for the review, @jasnell. I agree that changing MessageEvent.source makes this a semver-major change. Do you have any particular approach in mind for exposing the sender information while preserving the existing semantics of MessageEvent.source? I'd like to make sure I'm exploring all feasible directions that best align with Node's API design.

[jdmarshall]

@jasnell Source currently resolves to null. Why would this a breaking change when there is nobody in the forest to hear the tree fall?