Add version number to default font URLs on jsdelivr

[dpvc]

This PR adds version numbers to the URLs for fonts when they come from cdn.jsdeliver.net (the default). Without this, MathJax will load the latest font version, even if MathJax is not the latest version. This can lead to version mismatch warnings, and potentially to failures if the latest version has incompatible changes with the version being used.

This also adds support for inserting the Mathjax version number into the fontPath configuration value, so that if you move to a different CDN, you can include the version number in the appropriate place for that CDN.

This should improve the stability of MathJax as we make more v4 releases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (95faa0c) to head (fd39984).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1423     +/-   ##
==========================================
  Coverage    86.51%   86.51%             
==========================================
  Files          340      340             
  Lines        85993    86001      +8     
  Branches      4825     3186   -1639     
==========================================
+ Hits         74397    74405      +8     
  Misses       11596    11596             

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[zorkow]

lgtm.