Update to MathJax 4, add more configuration options (font, extensions)

[kno10]

This is not an urgent or immediate issue, but a mid-term TODO likely for the next year. I am already using MathJax v4.0.0-beta.4, and have positive experiences with it. A big plus of version 4 is the improved font support, including a Fira Math font that works well with Fira font. I directly went for version 4 because I have been using it on my web site before and did not want two different versions.

• [ ] wait for the final release, or even some further bug fix release?
• [ ] add easy-to-use configuration options for choosing the mathjax font (and -location)
• [ ] clean up mathjax loading (which differs from deck and non-deck files, some templates always load from vendor, while the plugin loads from options with a fallback to jsdelivr)
• [ ] make mathjax extension autoloading configurable

[mbotsch]

Sounds good to me. Let's sit together and implement this once MathJax 4 is final.