Implement a no-dark-mode component. (mathjax/MathJax#3502)

[dpvc]

This PR creates a new ui/no-dark-mode component that removes the dark-mode colors for MathJax (other than for the menu, which doesn't hook into the MathJax._ variable, and so doesn't have a reasonable way to alter the styles).

There are a number of files that set up dark-mode colors, and they are in various different extensions. This component works by first checking if the extension is already loaded, and modifying the needed styles if so, otherwise it sets up a loader configuration parameter for code that runs when the extension loads, and removes the styles at that point. The data needed to do this is stored in an array (giving the MathJax._ location to check for the extension being loaded, the name of the extension if not, and the code to run to remove the styles). The explorer adjustment also hooks into the startup handler processing in order to remove some styles, since they are in a mixing that can't be adjusted directly.

This component only works within the MathJax Components framework, so can't be used with direct module access (as the MathJax._ variable is not available there). If support for this in server-side processing is needed, we can address that as the need arrises.

Resolves issue mathjax/MathJax#3502.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (95faa0c) to head (60d2484).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1422   +/-   ##
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  Coverage    86.51%   86.51%           
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  Files          340      340           
  Lines        85993    85994    +1     
  Branches      4825     4825           
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+ Hits         74397    74398    +1     
+ Misses       11596    11573   -23     
- Partials         0       23   +23     

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