HTMLWriter: Change code block markup to resemble hugo chroma output

The highlight function from hugo already wraps the highlighted source code in
div > pre > code

e.g.

<div class="highlight">
<pre
style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4">
<code class="language-sh" data-lang="sh">echo hello world</code>
</pre>
</div>

So now we always delegate all that to the highlight function

org/testdata/example.html

@@ -63,9 +63,7 @@ and some lines of text
 <p>
 and another subitem
 </p>
-<code class="src src-sh">
-<pre>echo with a block</pre>
-</code>
+<div class="highlight"><pre>echo with a block</pre></div>
 </li>
 <li>
 <p>
@@ -166,14 +164,10 @@ auto link, i.e. not inside <code class="verbatim">\[[square brackets]\]</code> <
 </li>
 </ul>
 <h2>blocks</h2>
-<code class="src src-bash">
-<pre>echo a bash source block</pre>
-</code>
-<code class="src src-text">
-<pre>a source block without a language
+<div class="highlight"><pre>echo a bash source block</pre></div>
+<div class="highlight"><pre>a source block without a language
 and a second line
-and a third one</pre>
-</code>
+and a third one</pre></div>
 <pre class="example">\nan example block
 with multiple lines
 </pre>
@@ -280,9 +274,7 @@ and other elements
 <p>
 like blocks
 </p>
-<code class="src src-text">
-<pre>other non-plain</pre>
-</code>
+<div class="highlight"><pre>other non-plain</pre></div>
 </li>
 <li>
 <p>