Add popover light dismiss integration

I am adding a new HTML attribute called "popover" in
whatwg/html#8221

Part of this feature involves "light dismiss", where clicking outside of
an element with the popover attribute causes it to be closed. This
functionality was originally implemented by changing the event
dispatching system of the DOM, but it was recommended to move it here
instead:
whatwg/dom#1117 (comment)
https://chromium-review.googlesource.com/c/chromium/src/+/4023021

index.html

@@ -409,6 +409,8 @@ <h2>Firing events using the <code>PointerEvent</code> interface</h2>
                 <p>If the event is {{GlobalEventHandlers/pointerdown}}, the associated device is a direct manipulation device, and the target is an {{Element}},
                    then <a>set pointer capture</a> for this <code>pointerId</code> to the target element as described in <a>implicit pointer capture</a>.
 
+                <p>Run <a href="html.spec.whatwg.org/#popover-light-dismiss">light dismiss open popovers</a> given the event and target.</p>
+
                 <p>Fire the event to the determined target. The user agent SHOULD treat the target as if the pointing device has moved over it for the purpose of <a data-cite="uievents/#events-mouseevent-event-order">ensuring event ordering</a> [[UIEVENTS]].</p>
 
                 <div class="note">Using the <a>pointer capture target override</a> as the target instead of the normal hit-test result may fire some boundary events, as defined by [[UIEVENTS]]. This is the same as the pointer leaving its previous target and entering this new capturing target. When the capture is released, the same scenario may happen, as the pointer is leaving the capturing target and entering the hit-test target.</div>