Add support for \P and others
Add support for \P and others

* https://phabricator.wikimedia.org/T107867 identifies
  that \P failes
* https://github.com/wikimedia/mathoid/blob/1ad83e4a673e3a9c3abd2efd7a43750532d179f0/test/files/mathjax-texvc/test-p.md
* identifies further problems with allowed texvc markup

This change ports fixes from the old Client-Side MathJax
extension to the current version.

wikimedia/MathJax,physikerwelt/MathJax,GerHobbelt/MathJax,GerHobbelt/MathJax