fix '\\' being converted to single backslash

Appears the default changed in 3.x
https://github.com/mathjax/MathJax/issues/2532

https://forums.ankiweb.net/t/double-backslashes-in-text-of-notes-are-turned-into-single-backslashes-on-display/9048

qt/aqt/data/web/js/mathjax.js

@@ -3,6 +3,7 @@ window.MathJax = {
         displayMath: [["\\[", "\\]"]],
         processRefs: false,
         processEnvironments: false,
+        processEscapes: false,
         packages: {
             "[+]": ["noerrors", "mhchem"],
         },