Chore: Add `Elixir` Syntax Highlighting

[nelsonic]

At present the syntax highlighting in the Book is not working for Elixir: https://dwyl.github.io/book/mvp/16-lists.html

It works fine on GitHub: https://github.com/dwyl/book/blob/main/src/mvp/16-lists.md

Looks like we need a custom highlight.js file: https://rust-lang.github.io/mdBook/format/theme/syntax-highlighting.html

Todo

• [ ] fix this for Elixir and SQL code. 🤞

[nelsonic]

Syntax highlighting working but fugly:

Sooo going to try a different theme: https://highlightjs.org/examples

[nelsonic]

Nah. Still nowhere near anything I would want to read ... https://dwyl.github.io/book/mvp/16-lists.html

Need help with this. 🙏

[nelsonic]

Gonna try: https://draculatheme.com/highlightjs

[nelsonic]

Ok, that backfired. 🤦‍♂️

Revert.

[nelsonic]

This looks "OK" and I'm happy enough for now to continue: https://dwyl.github.io/book/mvp/16-lists.html

Very happy for someone else with more time and a design sensibility to improve this. 🙏