add mutationobserver to fix disconnect build break

Co-authored-by: Copilot <copilot@github.com>

Author: stephanieyzhang

dom.bs

@@ -4121,7 +4121,7 @@ dictionary MutationObserverInit {
    or omitted.
   </dl>
 
- <dt><code><var>observer</var> . {{disconnect()}}</code>
+ <dt><code><var>observer</var> . {{MutationObserver/disconnect()}}</code>
  <dd>Stops <var>observer</var> from observing any mutations. Until the {{observe()}} method is used
  again, <var>observer</var>'s <a for=MutationObserver>callback</a> will not be invoked.