Provide the ability to add additional-js files above book.js (to support custom language highlights)

[rix0rrr]

Problem

This is a re-open of #1870. That issue was closed without addressing the underlying problem.

I want to add syntax highlighting for a custom, not super popular language (Alloy).

I can write my own custom JS file with:

hljs.registerLanguage(...);

But I cannot get mdbook to include my custom JS file before it initializes the page.

Proposed Solution

Include additional-js BEFORE book.js. Or provide a different key which can be used for scripts that should be included before book.js.

Notes

No response