[whatwg/dom] Proposal: add `children` option to `MutationObserverInit` (#905)

## Background

The `MutationObserverInit` has a `subtree` option that is *not* usable, unless `childList` or `attributes` are also defined.

The controversial part of this choice is that `attributes` inevitably involve *Element* only, while `childList` includes *Node* of any type.

As result, it is impossible to optimally observe `children` that are being added, when text, comments, or character data, are being manipulated, in a way or another, by libraries and whatsoever.

The workaround to fulfill an "*observe children only*" is to append any *Element* in the DOM and then `element.setAttribute('data-what', '')` so that a *MutationObserver* aware of `subtree` and `attributes` only, can verify the attribute is `data-what` and conside that node, instead of parsing every single kind of node that could land on a document/element.

## Proposal

Add a `children` *boolean* property to the `MutationObserverInit` to indicate that nothing else should bother the callback in charge of screening nodes, otherwise inevitably bloated, and slowed down, by checks such as `if (addedNode.nodeType === 1) ...` or similar.

Thanks for considering this improvement to one of the most successful DOM APIs in latter years.

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