Add popover default behavior for invoke targets

This specifies what buttons with an invoketarget pointing to an element
with `popover` should do, based on the Invokers proposal (#9625).

This introduces new specifications just within the "invoke target attribute activation behavior" algorithm such that:

 - If an `invoketarget` points to an element with `popover`
    - If the `invokeaction` is `auto` or `togglePopover`, try to toggle the popover
    - If the `invokeaction` is `hidePopover`, try to hide the popover
    - If the `invokeaction` is `showPopover`, try to show the popover

If the `invokeaction` is none of the above, then it will fall through
the logic and be passed over to the "invocation action algorithm" for
the invokee element, if present.

Things NOT covered in this commit that are included in the propoal (#9625):

- Default per element behaviours (invocation action algorithms) beyond
  popovers. These will be dealt with in subsequent commits adding each
  elements "invocation action algorithm".

- `interestaction` and `interesttarget`.

source

@@ -84988,9 +84988,71 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
    data-x="dom-invokeevent-invoker">invoker</code> set to <var>node</var>, and its <code
    data-x="dom-Event-cancelable">cancelable</code> attribute initialized to true.</p></li>
 
-   <li><p>If <var>notCancelled</var> is true and <var>invokee</var> has an associated
-   <span>invocation action algorithm</span> then run the <var>invokee's</var> <span>invocation
-   action algorithm</span> given <var>action</var>.
+   <li>
+    <p>If <var>notCancelled</var> is true:</p>
+
+    <ol>
+     <li>Let <var>shouldShowPopover</var> be false.
+
+     <li>Let <var>shouldHidePopover</var> be false.
+
+     <li><p>If <var>action</var> is an <span>ASCII case-insensitive</span> match for "<code
+     data-x="">togglePopover</code>" or "<code data-x="">auto</code>", then:</p>
+
+      <ol>
+
+       <li><p>If <var>invokee</var>'s <span>popover visibility state</span> is <span
+       data-x="popover-hidden-state">hidden</span>, then set <var>shouldShowPopover</var> to
+       true.</p></li>
+
+       <li><p>Otherwise set <var>shouldHidePopover</var> to true.</p></li>
+
+      </ol>
+
+     </li>
+
+     <li><p>if <var>action</var> is an <span>ASCII case-insensitive</span> match for "<code
+     data-x="">hidePopover</code>", and <var>invokee</var>'s <span>popover visibility
+     state</span> is <span data-x="popover-showing-state">showing</span>, then set
+     <var>shouldHidePopover</var> to true.
+
+     <li><p>if <var>action</var> is an <span>ASCII case-insensitive</span> match for "<code
+     data-x="">showPopover</code>", and <var>invokee</var>'s <span>popover visibility
+     state</span> is <span data-x="popover-hidden-state">hidden</span>, then set
+     <var>shouldShowPopover</var> to true.
+
+     <li><p>If <var>shouldShowPopover</var> is true, then:</p>
+
+      <ol>
+
+       <li><p><span>Assert</span>: <var>shouldHidePopover</var> is false.</p></li>
+
+       <li><p>Run the <span>show popover</span> given <var>invokee</var>, false, and <var>node</var>.</p></li>
+
+      </ol>
+
+     </li>
+
+     <li><p>Otherwise, if <var>shouldHidePopover</var> is true and the result of
+     running <span>check popover validity</span> given <var>popover</var>, false, false, and null
+     is true, then:</p>
+
+      <ol>
+
+       <li><p><span>Assert</span>: <var>shouldShowPopover</var> is false.</p></li>
+
+       <li><p>Run the <span>hide popover algorithm</span> given <var>invokee</var>, true, true, and false.</p></li>
+
+      </ol>
+
+     </li>
+
+     <li><p>Otherwise, if <var>invokee</var> has an associated <span>invocation action algorithm</span>
+     then run the <var>invokee's</var> <span>invocation action algorithm</span> given <var>action</var>.</p></li>
+
+    </ol>
+
+   </li>
 
   </ol>