Add \providecommand and tests

[dpvc]

This PR implements the \providecommand macro requested in mathjax/MathJax#3603. This is an easy addition to the newcommand package. I added a test for it as well.

Resolves issue mathjax/MathJax#3603.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.03%. Comparing base (c50078e) to head (3991366).
⚠️ Report is 25 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1555      +/-   ##
===========================================
- Coverage    87.03%   87.03%   -0.01%     
===========================================
  Files          390      390              
  Lines        88120    88122       +2     
  Branches      5045     5043       -2     
===========================================
+ Hits         76699    76700       +1     
+ Misses       11421    11402      -19     
- Partials         0       20      +20     

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[zorkow]

lgtm.