Add the DOM XPath interfaces from the WHATWG wiki

See also https://wiki.whatwg.org/wiki/DOM_XPath. Prose still needs to be added, tracked by #67. This is a one-time exception from the normal WHATWG Working Mode as there's a lot of benefit in hosting these interfaces in a standard as they're implemented in all engines, despite them not being fully defined and tested.

Co-Authored-By: Sam Sneddon <me@gsnedders.com>

dom.bs

@@ -9855,6 +9855,88 @@ the given value.
 
 
 
+<h2 id=xpath>XPath</h2>
+
+<p class=XXX><cite>DOM Level 3 XPath</cite> defined an API for evaluating <cite>XPath 1.0</cite>
+expressions. These APIs are widely implemented, but have not been maintained. The interface
+definitions are maintained here so that they can be updated when <cite>Web IDL</cite> changes.
+Complete definitions of these APIs remain necessary and such work is tracked and can be contributed
+to in <a href="https://github.com/whatwg/dom/issues/67">whatwg/dom#67</a>. [[DOM-Level-3-XPath]]
+[[XPath]] [[WEBIDL]]
+
+
+<h3 id=interface-xpathresult>Interface {{XPathResult}}</h3>
+
+<pre class=idl>
+[Exposed=Window]
+interface XPathResult {
+  const unsigned short ANY_TYPE = 0;
+  const unsigned short NUMBER_TYPE = 1;
+  const unsigned short STRING_TYPE = 2;
+  const unsigned short BOOLEAN_TYPE = 3;
+  const unsigned short UNORDERED_NODE_ITERATOR_TYPE = 4;
+  const unsigned short ORDERED_NODE_ITERATOR_TYPE = 5;
+  const unsigned short UNORDERED_NODE_SNAPSHOT_TYPE = 6;
+  const unsigned short ORDERED_NODE_SNAPSHOT_TYPE = 7;
+  const unsigned short ANY_UNORDERED_NODE_TYPE = 8;
+  const unsigned short FIRST_ORDERED_NODE_TYPE = 9;
+
+  readonly attribute unsigned short resultType;
+  readonly attribute unrestricted double numberValue;
+  readonly attribute DOMString stringValue;
+  readonly attribute boolean booleanValue;
+  readonly attribute Node? singleNodeValue;
+  readonly attribute boolean invalidIteratorState;
+  readonly attribute unsigned long snapshotLength;
+
+  Node? iterateNext();
+  Node? snapshotItem(unsigned long index);
+};
+</pre>
+
+
+<h3 id=interface-xpathexpression>Interface {{XPathExpression}}</h3>
+
+<pre class=idl>
+[Exposed=Window]
+interface XPathExpression {
+  // XPathResult.ANY_TYPE = 0
+  XPathResult evaluate(Node contextNode, optional unsigned short type = 0, optional XPathResult? result = null);
+};
+</pre>
+
+
+<h3 id=mixin-xpathevaluatorbase>Mixin {{XPathEvaluatorBase}}</h3>
+
+<pre class=idl>
+callback interface XPathNSResolver {
+  DOMString? lookupNamespaceURI(DOMString? prefix);
+};
+
+interface mixin XPathEvaluatorBase {
+  [NewObject] XPathExpression createExpression(DOMString expression, optional XPathNSResolver? resolver = null);
+  XPathNSResolver createNSResolver(Node nodeResolver);
+  // XPathResult.ANY_TYPE = 0
+  XPathResult evaluate(DOMString expression, Node contextNode, optional XPathNSResolver? resolver = null, optional unsigned short type = 0, optional XPathResult? result = null);
+};
+Document includes XPathEvaluatorBase;
+</pre>
+
+
+<h3 id=interface-xpathevaluator>Interface {{XPathEvaluator}}</h3>
+
+<pre class=idl>
+[Exposed=Window, Constructor]
+interface XPathEvaluator {};
+
+XPathEvaluator includes XPathEvaluatorBase;
+</pre>
+
+<p class=note>For historical reasons you can both construct {{XPathEvaluator}} and access the same
+methods on {{Document}}.
+
+
+
 <h2 id=historical>Historical</h2>
 
 <p>As explained in <a href="#goals">goals</a> this standard is a significant revision of various