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).
⚠️ Report is 7 commits behind head on develop.

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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[zorkow]

My only question is: Do we not also need a node-main.mjs for ESM?

[dpvc]

You are probably right, both versions should be able to use node-main.cjs if I add global.require = require to it. I'll check that out and update the PR if so.

[dpvc]

It turns out that yes, we do need the mjs version. There are two reasons: one is that when @mathjax/src/source is used, the files are taken from the components/mjs/node-main directory, and the node-main.js file there is considered an ESM file (the main package.json file is in effect, with type: module), so we can't use require() to load it. It might be possible to change node-main.js to node-main.cjs and node-main.cjs to node-main-start.cjs or some such thing, but that would complicate the build process, since that assumes a .js file. The webpack.,cjs` file might be able to be used to overcome this, but I don't think it is worth the effort. If you think otherwise, I can try it and see.

The other reason is to get the exports correct; the mjs file exports both MathJax and the init function, in order to make it compatible with v3 usage. This makes it work the same as how require() would.

So I think it is better to leave it as is rather than try to eliminate this one small file.