Update highlight.js - support syntax highlighting for more languages

[szabgab]

There are many issues and even PRs asking to add syntax highlighting support to other languages. Assuming we keep using highlight.js the only way to accomplish that is to upgrade highlight.js and include more language. However each addition language will increase the size of the highlight.js file while most books only need support for a very limited number of languages and formats. (e.g. unlikely to have a book that has code snippets in rust, ruby, scala, and swift. Except the test_book of mdbook)

Currently highlight.js is 137,537. If we download one that includes support for all the languages it will be 1,078,246 minified! We don't want every book to load that file.

Suggestion

My suggesting is that we generate a unique highlight.js file for every book that supports exactly the languages the book uses.

This will allow us to support every language highlight.js supports and keep the highlight.js file small. Maybe even smaller than what we have now.

Implementation

• Download the bundle from highlight.js and extract it in a folder called hl.
• It has a minimized JavaScript file that contains the core code and the code of each language.
• cp hl/highlight.min.js src/front-end/js/highlight.js
• We might need to pick one of the styles from the styles folder (though we might be able to keep the one we already have)
• During the book building phase we extract the list of languages used in the book.
• We extract the core and the languages from the source highlight.min.js file and build a new file including only the core and the selected languages and save that as highlight.js.

[szabgab]

Apparently we cannot upgrade highlight.js because #1622 and the goal is to switch to syntect #1652