Add code block styling

[beechnut]

Previously, code blocks looked squished, didn't have any syntax highlighting, didn't have line numbers, and didn't have clear filenames.

This commit:

• Adds margin to code blocks, both fenced and inline
• Adds baseline syntax highlighting (it looks ok, better than nothing)
• Adds non-copyable line numbers
• Adds style for separate filenames

This commit makes it work, it doesn't make it right.

This commit does NOT: add diff-language highlighting, add a copy button, or add non-copyable prompts.

Closes #3955