[markdown demo] Include javascript mode

Closes codemirror#6011

Author: marijnh

mode/markdown/index.html

@@ -8,6 +8,7 @@
 <script src="../../lib/codemirror.js"></script>
 <script src="../../addon/edit/continuelist.js"></script>
 <script src="../xml/xml.js"></script>
+<script src="../javascript/javascript.js"></script>
 <script src="markdown.js"></script>
 <style>
       .CodeMirror {border-top: 1px solid black; border-bottom: 1px solid black;}