Fix problem with Styles handling of margin. (mathjax/MathJax#3443)

[dpvc]

This PR fixes a problem where margin CSS styles aren't properly handled by the Styles object. This is because margin wasn't set in the connections array, so setting margin didn't propagate to margin-left, etc. This means that when margin-left was set earlier and then margin is set to '' later, the margin-left setting is not cleared. That lead to extra margin settings in the example given in mathjax/MathJax#3443.

This PR adds the margin connections, and adds more tests for margins, and for overriding connections when the main property is set.

It also checks that the left and right margins aren't zero when setting them for shifting the output in the CHTML Wrapper object. This is just to keep the CSS minimal.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.71%. Comparing base (afaf1ea) to head (1af7208).
⚠️ Report is 78 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/output/chtml/Wrapper.ts	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1362      +/-   ##
===========================================
- Coverage    86.72%   86.71%   -0.01%     
===========================================
  Files          337      337              
  Lines        84145    84155      +10     
  Branches      3140     4769    +1629     
===========================================
+ Hits         72971    72977       +6     
- Misses       11174    11178       +4     

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[zorkow]

lgtm.