It does have Julia support, it is just not included in the default bundle. Here is the julia style: https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.6.0/languages/julia.min.js and the julia-repl style: https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.6.0/languages/julia-repl.min.js
You can also use and improved version available here: Improved syntax highlighting for Julia on the web | Fredrik Ekre (see also Improved syntax highlighting for Julia on the web).