[whatwg/dom] Proposal: Make `DOMTokenList` a writable `setlike` (#884)

It's pretty blatantly very similar, and `setlike` objects have almost the same functionality.

- Setlikes use the native ES `Set` under the hood, which carries the same semantics as `DOMTokenList`s with regards to iteration order.
- `domTokenList.add(...items)` almost perfectly aligns with the native `setlike.add(item)`. The only difference is the return value (`undefined` instead of `this`) and the fact `domTokenList.add(...items)` accepts a list of tokens.
- `domTokenList.remove(...items)` similarly aligns with `setlike.delete(item)`, just in addition to the above, it goes by a different name.
- `domTokenList.contains(item)` is different from `setlike.has(item)` literally in name and identity only.
- `domTokenList.length` is different from `setlike.size` literally in name and getter identity only.
- For iterators, `DOMTokenList`s return indices for their keys, while setlikes delegate to the ES spec, which returns the values as the keys as well. For `values` and `@@iterator`, it's equivalent in practice.

There is one major caveat: this may require spec changes either to WebIDL, the DOM, or both to migrate `add`.

Not a simple change and might just be a pipe dream, but I feel it's definitely something worth exploring.

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