Remove Banzai::Renderer::CommonMark::HTML

Summary

This issue was created from this discussion https://gitlab.com/gitlab-org/gitlab/-/issues/263114#note_428271807.

We currently implement a custom commonmarker rendering class Banzai::Renderer::CommonMark::HTML which has the sole purpose of producing fenced code blocks in a format that renders nicely on GitLab.com (see this code comment).

Problem

@toupeira noticed https://gitlab.com/gitlab-org/gitlab/-/issues/263114#note_428271807 an issue https://github.com/gjtorikian/commonmarker/issues/112 that mentioned that doing so makes a renderer that is an "all or nothing" override of the C class with Ruby.

From https://github.com/gjtorikian/commonmarker/issues/112#issue-545161219, the style of rendering benchmarked as 10x slower (commonmarker_our_custom_renderer) is how we currently render markdown.

Example code block styles

Our Banzai::Filter::MarkdownEngines::CommonMark class currently outputs code blocks like this:

<pre><code lang="LANG">some code\n</code></pre>

GitHub-style looks like this:

<pre lang="LANG"><code>some code\n</code></pre>

And default Markdown style looks like this:

<pre><code class="language-LANG">puts 1\n</code></pre>

Improvements

If we can delete Banzai::Renderer::CommonMark::HTML we might get ~10x speed up improvement in rendering markdown.

We could either:

1. Move the customising of the fenced code block that Banzai::Renderer::CommonMark::HTML produces into another part of the Banzai pipeline.
2. Modify however GitLab renders fenced code blocks so that it works with either GitHub-style code fences or CommonMarker's default rendering of fenced code blocks.

Risks

We want to make sure that cached markup continues to work.

Involved components

TBD.