Code blocks should be rendered as <pre><code> instead of just <pre>

[berberman]

IIUC currently Block.toHtml renders .code blocks using only a <pre> container:

verso/src/verso/Verso/Doc/Html.lean

Line 172 in 1e49d3d

| .code content => pure #[{{ <pre> {{ content }} </pre>}}]

This breaks client-side highlighters like Highlight.js and some CSS themes which expect the standard structrue. I think it would be great to reuse some existing highlighters as a catch-all highlighting on the top of define_lexed_text defined languages.