Restore <xmp> to the "special" elements list

This was removed, apparently inadvertently, in
77c3dc4.

<xmp> is still handled specially in the "in body" insertion mode, in
the fragment serialization algorithm, and in the HTML fragment parsing
algorithm.  Further, because it was (previously) a member of the
"special" set, it was handled differently by the adoption agency
algorithm and by the "in body" insertion mode rules for <li>, <dd> and
<dt> start tags and "any other end tag".

source

@@ -100820,8 +100820,8 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
     <code>style</code>, <code>summary</code>, <code>table</code>, <code>tbody</code>,
     <code>td</code>, <code>template</code>, <code>textarea</code>, <code>tfoot</code>,
     <code>th</code>, <code>thead</code>, <code>title</code>, <code>tr</code>, <code>track</code>,
-    <code>ul</code>, <code>wbr</code>; <span>MathML <code>mi</code></span>, <span>MathML
-    <code>mo</code></span>, <span>MathML <code>mn</code></span>, <span>MathML
+    <code>ul</code>, <code>wbr</code>, <code>xmp</code>; <span>MathML <code>mi</code></span>,
+    <span>MathML <code>mo</code></span>, <span>MathML <code>mn</code></span>, <span>MathML
     <code>ms</code></span>, <span>MathML <code>mtext</code></span>, and <span>MathML
     <code>annotation-xml</code></span>; and <span>SVG <code>foreignObject</code></span>, <span>SVG
     <code>desc</code></span>, and <span>SVG <code>title</code></span>.</p> <!-- we could actually
@@ -119559,6 +119559,7 @@ INSERT INTERFACES HERE
   Bruce Lawson,
   Bruce Miller,
   Bugs Nash,
+  C. Scott Ananian,
   C. Williams,
   Cameron McCormack,
   Cameron Zemek,