Use subclass for marked renderer

This better matches how things are done in Marked, and allows us to user `super` for cases other than images.

Co-authored-by: Oliver Byford <oliver.byford@digital.cabinet-office.gov.uk>

lib/marked-renderer.js

@@ -1,13 +1,13 @@
 // A custom renderer that avoids wrapping <img> tags in <p> tags
 const { marked } = require('marked')
 
-const markedRenderer = new marked.Renderer()
-
-markedRenderer.paragraph = function (text) {
-  if (text.startsWith('<img')) {
-    return `${text}\n`
+class DesignSystemRenderer extends marked.Renderer {
+  paragraph (text) {
+    if (text.startsWith('<img')) {
+      return `${text}\n`
+    }
+    return super.paragraph(text)
   }
-  return `<p>${text}</p>\n`
 }
 
-module.exports = markedRenderer
+module.exports = new DesignSystemRenderer()