Fenced code blocks should add class to `code` rather than `pre`, matching the HTML best practice

Spec
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@mathias, isn’t the best practice to utilise both, like <pre><code lang="mul" class="language-javaScript">? The WHATWG’s HTML5 LS and IETF’s BCP 47 appear to recommend this (with type="text/x.javaScript" also appended, in scripts).