Add tab key support to contextual menu, and add an aria-label (mathjax/MathJax#3609)

[dpvc]

This PR adds a check for the Tab key in the key event handler and unposts the message when that occurs. It also adds an ARIA label to the menu, as requested in mathjax/MathJax#3609. As we know, the menu code needs an overhaul, which should be done after it is moved into the MathJax-src repository.

Resolves issue mathjax/MathJax#3609.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.13%. Comparing base (91b8038) to head (83b3071).
⚠️ Report is 17 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1569   +/-   ##
========================================
  Coverage    87.12%   87.13%           
========================================
  Files          392      392           
  Lines        89187    89213   +26     
  Branches      5063     5067    +4     
========================================
+ Hits         77706    77732   +26     
  Misses       11481    11481           

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[zorkow]

Just one left over logging. Otherwise fine.