[DX] Introduce a new `#` placeholder for `format_string()`, that formats user-facing text as code

I'm splitting this request from #6190, since it seems easier to implement and more straight-forward.

I would like us to introduce # as a new type of placeholder for the format_string() function, which would cause the replaced text to be wrapped in HTML tags that allow it to be shown as a code snippet. It should basically be mimicking the current % placeholder, but use a different HTML tag (</code> instead of <em>).