add syntax/error highlighting and fix editor bug (#4723)

Author: ebeetles

index.html

@@ -52,7 +52,13 @@
     <script src="lib/wheelnav.js" defer></script>
     <script src="lib/abc.min.js" defer></script>
     <script src="lib/codejar/codejar.min.js" defer></script>
-    <script src="lib/codejar/highlight.pack.js" defer></script>
+    <script src="//cdnjs.cloudflare.com/ajax/libs/highlight.js/11.7.0/highlight.min.js"></script>
+    <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/11.7.0/styles/atom-one-dark.min.css">
+    <script>
+        hljs.highlightAll();
+        <code class="language-javascript">
+        </code>
+    </script>
     <script src="lib/astring.min.js" defer></script>
 
     <script src="lib/acorn.min.js" defer></script>