CodeBlock line numbers should not be implemented using a before pseudo element.

The use of a before pseudo element means that a unique class is generated for every line of code. This could be simplified by simply rendering an actual DOM element that is not user selectable.

_{Sent with GitHawk}

Start by locating the CodeBlock component and inspect how line numbers are currently produced with a before pseudo-element and per-line classes. Replace that mechanism with an actual non-selectable DOM element, then verify that line numbers render correctly without generating a unique class for every code line.

Written by the indexing model from the issue text.