Change default font to mathjax-newcm

[dpvc]

This PR changes the default front from mathjax-modern to mathjax-newcm, which has more coverage and is slightly darker. The lightness of mathjax-modern was one of the complaints with v4.

You need to do pnpm install to get the new font installed, and need the lab-extension-update branch in the MathJax-dev repository, and do npm install there as well, then relink node_modules/mathjax-full to your current MathJax-src repository.