Book: Code block line numbers

[azriel91]

Currently the book uses highlightjs-line-numbers.js@2.8.0, which:

• Breaks code snippet copying: As it uses <table> elements, it causes the copied code to contain extra newlines.
• Also necessitates a lot of hacky CSS rules so that mdbook's table styling doesn't interfere with the code blocks' styling.

Probably worth writing the same functionality without using <table> elements.

Figure out how to add <span>s for each line in the code block as a hljs plugin, then CSS counters to add the counter.

Links:

• highlight.js
• wcoder/highlightjs-line-numbers.js
• [mdn] counter-increment
• [blog] using-css-to-add-line-numbering
• [so] create-line-numbers-on-pre-with-css-only

[azriel91]

Closed by #163 in commit fcc0cb5d.

Source code of the highlightjs line numbers is at: https://github.com/azriel91/highlightjs-line-numbers.js/blob/feature/mdbook-support/src/highlightjs-line-numbers.js

Permanent solution would be solved in: https://github.com/rust-lang/mdBook/issues/1303