Improve code blocks.

[sustained]

We can probably improve our code blocks.

Right now we just wrap in a pre and let hljs do its magic but we could maybe wrap it in a component allowing us to provide various niceties such as:

• Allowing toggling of highlighting.
• Selecting/copying to clipboard.
• "Open in CodePen" or similar functionality.
• Showing a nice header with the language name.
• etc.