Fix magnification of HTML embedded in MathML or TeX.

[dpvc]

This PR fixes a problem with the keyboard or mouse magnification of expressions that include HTML embedded in the MathML or TeX expressions. The HTML was not being properly enlarged in the magnification window. This is due to the use of px font sizing in the CHTML output, and a pad interaction with the scaling in SVG output.

The solution for CHTML is to override the getFontStyles() method to use a scaling in em units (in this case, the parent elements already do the relative scaling, so 1em keeps it at that scale). For SVG, setting font-size to initial and leaving the px values seems to do the trick.

[codecov]

Codecov Report

❌ Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.70%. Comparing base (9ef060f) to head (953b69b).

Files with missing lines	Patch %	Lines
ts/output/chtml/Wrappers/semantics.ts	55.55%	4 Missing ⚠️
ts/output/svg/Wrappers/semantics.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1530      +/-   ##
===========================================
- Coverage    86.70%   86.70%   -0.01%     
===========================================
  Files          340      340              
  Lines        86264    86273       +9     
  Branches      3246     4906    +1660     
===========================================
+ Hits         74798    74802       +4     
+ Misses       11466    11451      -15     
- Partials         0       20      +20     

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