Allow rendering content to CDATA section

[MFAshby]

Related a bit to #101

Use case: I want to generate an RSS feed with the full content of my posts embedded in the RSS XML file. Here is my superhtml template:

<rss version="2.0" 
  xmlns:atom="http://www.w3.org/2005/Atom"
  xmlns:content="http://purl.org/rss/1.0/modules/content/">
  <channel>
    <title>Home on mfashby.net</title>
    <link>https://mfashby.net/</link>
    <description>Recent content in Home on mfashby.net</description>
    <language>en-gb</language>
    <lastBuildDate>Sun, 03 Mar 2024 19:31:27 +0000</lastBuildDate>
    <atom:link href="https://mfashby.net/index.xml" rel="self" type="application/rss+xml"/>
    <item :loop="$page.subpages()">
      <title :text="$loop.it.title"></title>
      <link :text="$loop.it.link()"></link>
      <pubDate :text="$loop.it.date.formatHTTP()"></pubDate>
      <guid isPermaLink="true" :text="$loop.it.link()"></guid>
      <description :text="$loop.it.description"></description>
      <content:encoded :html="$loop.it.content()"></content:encoded> 
    </item>
  </channel>
</rss>

however, valid HTML is not necessarily valid XML; so in order for the rendered RSS file to remain valid XML it might be necessary to escape the content HTML. The easiest way to achieve this might be to render the element content into a CDATA section. See https://www.rssboard.org/rss-profile#namespace-elements-content-encoded, but I did not find a way to do this in Zine/SuperHTML.