Re: [whatwg/dom] Allow `MutationObserver` to observe across `<slot>`s (Issue #1415)

sorvell left a comment (whatwg/dom#1415)

This would be a useful addition. 

Right now, although it's a little clunky, I think you can leverage `slotchange` to do most of this:

```js
const myObserver = new MutationObserver(callback);

let previousAssigned = new Set();

slot.addEventListener('slotchange', (e) => {
  myObserver.disconnect();
  const assigned = new Set(this.observeSlot.assignedElements({flatten: true}));
  assigned.forEach(el => {
    myObserver.observe(el, {attributes: true});
  });

  // if needed, an process slot changes similar to other mutation observer childList mutations...
  const addedNodes = Array.from(assigned.difference(previousAssigned));
  const removedNodes = Array.from(previousAssigned.difference(assigned));
  callback([{target: slot, addedNodes, removedNodes}])

  previousAssigned = assigned;
})
```

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