Use cjs for node-main-setup so it works in bun as well as node. (mathjax/MathJax#3476)

[dpvc]

This PR changes node-main-setup.mjs to be node-main-setup.cjs so that the import order is respected in the bun environment (works around what I think is a bug in bun).

This doesn' have to go into 4.1, but it is pretty straight-forward, so perhaps we can squeeze it in.

Resolves issue mathjax/MathJax#3476.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.49%. Comparing base (a14d976) to head (49d8b61).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1397      +/-   ##
===========================================
- Coverage    86.49%   86.49%   -0.01%     
===========================================
  Files          340      340              
  Lines        85813    85811       -2     
  Branches      3177     4815    +1638     
===========================================
- Hits         74220    74218       -2     
+ Misses       11593    11570      -23     
- Partials         0       23      +23     

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