Fix path handling to work with Windows and add tests for it

[dpvc]

This PR makes it possible to run MathJax via node in Windows (without needing to use a unix subsystem). The main issue is, as usual, the use of \ rather than / in Windows path names, so we introduce a new function in the context object that converts Windows (absolute) paths to unix-style paths when needed, and use this whenever __dirname or import.meta.url or similar values are used. That way, the management of paths can use / as it currently does.

In addition, the context setup for context.os now handles the situation within node as well (it used to always return undefined). This is done using the process.platform value to make to the generic Unix, Windows, and MacOS values. I wasn't sure how to treat cygwin, but have decided to set it to Windows, since it should not hurt to do the path adjustment there, and my understanding is that cygwin can use Windows paths, so it might be useful to do the translation anyway.

The context tests are adjusted for the new values, and I've added several new files to handle testing Windows and to get full coverage in the tests.

This resolves issue mathjax/MathJax#3439.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.72%. Comparing base (afaf1ea) to head (f98da55).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1361     +/-   ##
==========================================
  Coverage    86.72%   86.72%             
==========================================
  Files          337      337             
  Lines        84145    84174     +29     
  Branches      3140     4776   +1636     
==========================================
+ Hits         72971    73000     +29     
  Misses       11174    11174             

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