Fix #99 by adding Element.prototype.closest

[farsightsoftware]

Went back and forth about whether to use el.matches(selectorString) or go straight to this.ownerDocument!._nwapi.match, but if you run this code on a browser, you don't see closest calling matches:

<div>
  <span>
    <em id="target"></em>
  </span>
</div>
<script>
const t = document.getElementById("target");
const m = t.matches;
t.matches = function (str) {
  console.log("***************");
  return m.call(this, str);
};
console.log("closest");
console.log(t.closest(".x"));
console.log("matches");
console.log(t.matches("#target"));
</script>

So went with _nwapi.match.

[b-fuze]

Looks good, the only thing that comes to mind is if someone is using the Sizzle engine selector (which was required at one point for Deno Deploy, but afaik isn't required anymore—though it can be explicitly enabled by the user)

[b-fuze]

Nvm, Sizzle also has match. This is good to go