Allow font family, weight, and style CSS to be inherited by token elements

[dpvc]

For an expression like

<mrow style="font-family: arial">
  <mn>1</mn>
  <mo>+</mo>
  <mn>2</mn>
</mrow>

the font family should affect all three elements that it contains. But currently in MathJax, that is not the case.

This PR fixes that issue and propagates the font family, weight, and style CSS styles of a container to its child nodes. For an mstyle element, the fontfamily, fontweight and fontstyle are also propagated. For an mi element with a single character content, the style will be set to italic for the specified font automatically.

This inheritance is accomplished by using the setInheritedAttributes() method. The font-family, font-weight, and font-style, if specified, are moved to the inherited fontfamily, fontweight, and fontstyle attributes (these are inherited values, not explicit attributes, so don't show up in the MathML output directly). The output jax wrapper's getVariant() method then picks these up and uses them when determine the math variant to use (which can be an explicit font). This handling is simplified a bit as well.

[codecov]

Codecov Report

❌ Patch coverage is 74.44444% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.12%. Comparing base (91b8038) to head (341626d).
⚠️ Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/core/MmlTree/MmlNode.ts	50.00%	23 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1574      +/-   ##
===========================================
- Coverage    87.12%   87.12%   -0.01%     
===========================================
  Files          392      392              
  Lines        89187    89280      +93     
  Branches      5063     3378    -1685     
===========================================
+ Hits         77706    77784      +78     
- Misses       11481    11496      +15     

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[zorkow]

lgtm.