[dom] element.furthest(selector) proposal (#163)

It's probably really rare to need that

```
// returns the last element matching a selector from the reference element inside a container element 
// div#bar
//   div#foo1.foo
//     article
//       div.foo
//         span#qux
// qux.furthest('.foo', bar) -> foo1
Node.prototype.furthest = function(selector, el = document){ // overloading dom prototypes can be bad :/
 //if (el.matches(selector)) return el;
 let current = this.closest(selector);
 if (!current) return null;
 let _el = current.closest(el.tagName);// for avoiding having to call .closest until leaving el
  while( _el && _el !== el) {
   current = _el.parentNode;
   _el = current.closest(el.tagName)
  }
  return current;
}
```

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/163