Add highlight.js for syntax highlighting

(Reframing the issue based on the discussion)

Syntax highlighting with highlight.js will work with the code blocks that ui.markdown() produces, or any <pre><code> block. It requires:

• A JS file
• A CSS file, which varies by theme
• A script tag to call highlightAll() on the page

We probably want to set the theme globally, like with bootswatch: the highlight.js interface does not seem to be set up to apply different style to parts of the page, nor does that seem worth the effort to support.

We need every ui.markdown() to call highlightAll(), like @wch shows below, so that dynamically generated UI gets updated.

If I understand correctly then, we either need to always add the highlight.js JS code, or only add the highlightAll call if highlight.js has been added to the page, or otherwise shim the Shiny JS so that hljs.highlightAll(); doesn't error if it is invoked in a markdown block but someone hasn't added highlighting/theming to the page.

Start by tracing the ui.markdown() output and how page-level JavaScript and CSS assets are included. Review the proposed highlightAll() call and theme handling, then confirm the integration covers both initial and dynamically generated markdown code blocks without errors when highlighting is absent.