Re: [WICG/webcomponents] Need a callback for when children changed or parser finished parsing children (#809)

It would help avoid the partial node insertions and nested parent checks, but would still require us to create our own queueing/callback system when `closingTagParsed === false`.

Possible shapes of the solution I had in mind:

### MutationObserver closingTag event
To avoid backward compatibility issues, opt-in with `const config = { closingTagEvent: true };` which would either emit on all closing tags, or only on nodes where `closingTagParsed === false` initially to reduce events emitted.

This might present a performance bottleneck with potentially double the callbacks, but would otherwise be simple to use by just watching for `closingTagParsed === true` in existing handler logic to only handle complete tags.

### onClosed callback
Introduce an explicit callback on either a node or a mutation:
```
if (record.closingTagParsed === false) {
  record.onClosed((node) => {
    document.body.appendChild(node);
  });
}
// or
if (record.closingTagParsed === false) {
  record.target.onClosed(() => {
    // Handle complete node
    document.body.appendChild(record.target);
  });
}
```

This could introduce other performance bottlenecks but is more explicit.  If the node is already complete it would fire immediately.  Otherwise it would fire once the node closing tag is parsed.


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