Update font-size macros to correct LaTeX values and make fontsizev3 package for backward compatibility

[dpvc]

This PR fixes several incorrect sizes for the font-sizing commands (\tiny and \Tiny were switched, and the larger sizes were all off by one). It also adds \SMALL and \Small, which were missing, and \footnotesize, since several people have asked for that. Finally, since the largest size got shifted down one size, I've added \HUGE, which seems that it should have been there, but isn't, at least in the amsart class that is the basis for MathJax's definitions. It does seem to be defined in several other packages, however.

Resolves issue mathjax/MathJax#3280.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.49%. Comparing base (5629a28) to head (1c64d74).
⚠️ Report is 23 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1449     +/-   ##
==========================================
  Coverage    86.49%   86.49%             
==========================================
  Files          340      340             
  Lines        86044    86049      +5     
  Branches      4833     3190   -1643     
==========================================
+ Hits         74425    74430      +5     
  Misses       11619    11619             

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[zorkow]

lgtm.