Define the onslotchange event handler IDL attribute

This introduces it to ShadowRoot in addition to GlobalEventHandlers, as
a counterpart to whatwg/html#4129. Closes
whatwg/html#3487.

dom.bs

@@ -3327,7 +3327,7 @@ run these steps:
   </ol>
 
  <li><p><a for=set>For each</a> <var>slot</var> of <var>signalSet</var>, <a>fire an event</a>
- named <code>slotchange</code>, with its {{Event/bubbles}} attribute set to true, at
+ named {{HTMLSlotElement/slotchange}}, with its {{Event/bubbles}} attribute set to true, at
  <var>slot</var>.
 </ol>
 
@@ -5752,6 +5752,7 @@ invoked, must return a new {{DocumentFragment}} <a>node</a> whose <a for=Node>no
 interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
   readonly attribute Element host;
+  attribute EventHandler onslotchange;
 };
 
 enum ShadowRootMode { "open", "closed" };
@@ -5779,6 +5780,11 @@ null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow roo
 <p>The <dfn attribute for=ShadowRoot><code>host</code></dfn> attribute's getter must return the
 <a>context object</a>'s <a for=DocumentFragment>host</a>.
 
+<p>The <dfn attribute for=ShadowRoot><code>onslotchange</code></dfn> attribute is an
+<a>event handler IDL attribute</a> for the
+<dfn for=ShadowRoot export><code>onslotchange</code></dfn> <a>event handler</a>, whose
+<a>event handler event type</a> is {{HTMLSlotElement/slotchange}}.
+
 <hr>
 
 <p>In <dfn export id=concept-shadow-including-tree-order>shadow-including tree order</dfn>, is