Add Elixir to list of supported languages in code blocks

[jakoubek]

Describe the feature you'd like

I'd like syntax highlighting for the Elixir programming language in code blocks.

Describe the benefits this would bring to existing BookStack users

Be able to read Elixir code blocks more easily.

Can the goal of this request already be achieved via other means?

No

Have you searched for an existing open/closed issue?

• [X] I have searched for existing issues and none cover my fundamental request

How long have you been using BookStack?

1 to 5 years

Additional context

There is "elixir" in highlight.js : https://github.com/highlightjs/highlight.js/blob/main/src/languages/elixir.js

And yes, there is a ticket mentioning adding support for Elixir in the editor (#2125), but in the code editor it's not available nevertheless.