Update key explorer to use dialog element for help dialog. (mathjax/MathJax#3405)

[dpvc]

This PR allows the KeyExplorer to use a <dialog> element when the browser supports that, and uses the current method when that isn't available. In the latter case, we still create a dialog element, but it acts like a custom element in that case, since it isn't implemented directly.

This resolve the z-index issue reported in mathjax/MathJax#3405.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.72%. Comparing base (efcdd46) to head (b92c329).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1334   +/-   ##
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  Coverage    86.72%   86.72%           
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  Files          337      337           
  Lines        84145    84145           
  Branches      4769     4769           
========================================
  Hits         72971    72971           
  Misses       11151    11151           
  Partials        23       23           

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