Add onslotchange to GlobalEventHandlers

Fixes part of #3487; what remains is to add it to ShadowRoot.

source

@@ -11700,6 +11700,7 @@ https://software.hixie.ch/utilities/js/live-dom-viewer/?%3C%21DOCTYPE%20HTML%3E%
    <li><code data-x="handler-onseeked">onseeked</code></li>
    <li><code data-x="handler-onseeking">onseeking</code></li>
    <li><code data-x="handler-onselect">onselect</code></li>
+   <li><code data-x="handler-onslotchange">onslotchange</code></li>
    <li><code data-x="handler-onstalled">onstalled</code></li>
    <li><code data-x="handler-onsubmit">onsubmit</code></li>
    <li><code data-x="handler-onsuspend">onsuspend</code></li>
@@ -92852,6 +92853,7 @@ typedef <span>OnBeforeUnloadEventHandlerNonNull</span>? <dfn>OnBeforeUnloadEvent
     <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onseeked">onseeked</code></dfn> <td> <code data-x="event-media-seeked">seeked</code>
     <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onseeking">onseeking</code></dfn> <td> <code data-x="event-media-seeking">seeking</code>
     <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onselect">onselect</code></dfn> <td> <code data-x="event-select">select</code> <!-- widely used --> <!-- [CSSOM] -->
+    <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onslotchange">onslotchange</code></dfn> <td> <code data-x="event-slotchange">slotchange</code>
     <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onstalled">onstalled</code></dfn> <td> <code data-x="event-media-stalled">stalled</code>
     <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onsubmit">onsubmit</code></dfn> <td> <code data-x="event-submit">submit</code> <!-- widely used -->
     <tr><td><dfn data-export="" data-dfn-type="attribute" data-dfn-for="HTMLElement,Document,Window,GlobalEventHandlers"><code data-x="handler-onsuspend">onsuspend</code></dfn> <td> <code data-x="event-media-suspend">suspend</code>
@@ -93035,6 +93037,7 @@ typedef <span>OnBeforeUnloadEventHandlerNonNull</span>? <dfn>OnBeforeUnloadEvent
   attribute <span>EventHandler</span> <span data-x="handler-onseeked">onseeked</span>;
   attribute <span>EventHandler</span> <span data-x="handler-onseeking">onseeking</span>;
   attribute <span>EventHandler</span> <span data-x="handler-onselect">onselect</span>;
+  attribute <span>EventHandler</span> <span data-x="handler-onslotchange">onslotchange</span>;
   attribute <span>EventHandler</span> <span data-x="handler-onstalled">onstalled</span>;
   attribute <span>EventHandler</span> <span data-x="handler-onsubmit">onsubmit</span>;
   attribute <span>EventHandler</span> <span data-x="handler-onsuspend">onsuspend</span>;
@@ -122281,6 +122284,12 @@ interface <dfn>External</dfn> {
      <td> <code data-x="event-select">select</code> event handler
      <td> <span data-x="event handler content attributes">Event handler content attribute</span>
 
+    <tr>
+     <th id="ix-handler-onslotchange"> <code data-x="">onslotchange</code>
+     <td> <span data-x="handler-onslotchange">HTML elements</span>
+     <td> <code data-x="event-slotchange">slotchange</code> event handler
+     <td> <span data-x="event handler content attributes">Event handler content attribute</span>
+
     <tr>
      <th id="ix-handler-onstalled"> <code data-x="">onstalled</code>
      <td> <span data-x="handler-onstalled">HTML elements</span>
@@ -123049,6 +123058,12 @@ INSERT INTERFACES HERE
      <td> Form controls
      <td> Fired at form controls when their text selection is adjusted (whether by an API or by the user)
 
+    <tr> <!-- slotchange -->
+     <td> <dfn data-dfn-type="event" data-dfn-for="HTMLSlotElement" data-export=""><code data-x="event-slotchange">slotchange</code></dfn>
+     <td> <code>Event</code>
+     <td> <code>slot</code> elements
+     <td> Fired at <code>slot</code> elements when their <span>assigned nodes</span> change
+
     <tr> <!-- storage -->
      <td> <dfn data-dfn-type="event" data-dfn-for="Window" data-export=""><code data-x="event-storage">storage</code></dfn>
      <td> <code>StorageEvent</code>