Add readonly getSelection().direction property.

The return value matches the defintion of https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-textarea/input-selectiondirection

index.html

@@ -199,6 +199,7 @@ <h2>
           readonly attribute boolean isCollapsed;
           readonly attribute unsigned long rangeCount;
           readonly attribute DOMString type;
+          readonly attribute DOMString direction;
           Range getRangeAt(unsigned long index);
           undefined addRange(Range range);
           undefined removeRange(Range range);
@@ -290,6 +291,17 @@ <h2>
             `"Range"` otherwise.
           </p>
         </dd>
+        <dt>
+          <dfn>direction</dfn>
+        </dt>
+        <dd>
+          <p>
+            The attribute must return `"none"` if [=this=] is <a>empty</a> or
+            this selection is <a>directionless</a>. `"forward"` if this selection's
+            direction is <a>forwards</a> and `"backward"` if this selection's
+            direction is <a>backwards</a>.
+          </p>
+        </dd>
         <dt>
           <dfn>getRangeAt()</dfn> method
         </dt>