Backward compatibility for mathjax-full package

[dpvc]

This branch implements a backward-compatibility definition for the mathjax-full package that could be used to make mathjax-full to @mathjax/src. This is the idea I had mentioned to Peter in mathjax/MathJax#3390. It works well, and would make it easier to upgrade to MathJax v4 for those already using mathjax-full. But perhaps we don't want to support that, and should force everyone to change to @mathjax/src to use v4. This could remain as a separate branch and not be merged, as a means of keeping this separate. Comments?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (4124786).
Report is 45 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1296   +/-   ##
========================================
  Coverage    86.71%   86.71%           
========================================
  Files          337      337           
  Lines        83979    83979           
  Branches      4750     4750           
========================================
  Hits         72826    72826           
  Misses       11130    11130           
  Partials        23       23           

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[dpvc]

In face-to-face meeting, we decided not to make this package, so I'm closing the PR. But I'm leaving the branch for now as I may want to remember the package.json that does this.