[whatwg/dom] Could a property be exposed so I can observe `.stopImmediatePropagation()`? (#958)

I'm looking at this:

- `.preventDefault()` → `.defaultPrevented`
- `.stopPropagation()` → `.cancelBubble`
- `.stopImmediatePropagation()` → ???

I was working on a function like this, to combine attributes objects in a virtual DOM setting to allow really flexible attributes composition without much work, but as nothing like that `immediatePropagationStopped` exists (I'm not bound to that name in any way - it *is* a bit long), I instead have to rely on some property hacking (basically replacing it with a weak map + overridden method), which for anything particularly spammy, would become a major perf concern really quick.

```js
function combine(...attrsObjects) {
    const handlers = Object.create(null)
    const values = Object.create(null)

    for (const attrs of attrsObjects) {
        for (const [key, value] of Object.entries(attrs)) {
            if (key.startsWith("on")) {
                (handlers[key] || (handlers[key] = [])).push(value)
            } else {
                values[key] = value
            }
        }
    }

    for (const [key, list] of Object.entries(handlers)) {
        values[key] = e => {
            for (const func of list) {
                if (func(e) === false) e.preventDefault()
                if (e.cancelBubble || e.immediatePropagationStopped) return
            }
        }
    }

    return values[key]
}
```

(I'm using this in the wild, BTW. This isn't exactly academic code here.)

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