Prevent webpacking from producing invalid names for region stylesheets

[dpvc]

This PR fixes a problem with the id attribute for the style-sheets used for the various Region classes. It currently is based on the class's name property, which should be the class name, but when webpacked the name can be changes to some random characters, and it is possible for that to produce an invalid element id. E.g., I had one of the region classes be named $E, and that lead to an error in the browser for MJX-$E-styles being used as the id attribute.

The solution used here is to use the className property rather than name, since that is something like MJX_hover and won't be changed by webpack.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.12%. Comparing base (91b8038) to head (59bdc3d).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1565     +/-   ##
==========================================
  Coverage    87.12%   87.12%             
==========================================
  Files          392      392             
  Lines        89187    89187             
  Branches      5063     3370   -1693     
==========================================
  Hits         77706    77706             
  Misses       11481    11481             

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