Add fontExtensions configuration option. (mathjax/MathJax#3410)

[dpvc]

This PR adds a fontExtensions configuration that makes it easier to specify a font extension to load (e.g., mathjax-euler). It sets up the extraLoads package data to make sure the extension loads after the font itself. The current method of including the extension in the loader.load array doesn't work properly as it can be loaded before the font arrives. That can be handled by setting up dependencies and paths in the loader configuration, but it is harder than it should be, so this makes it easy.

Resolves issue mathjax/MathJax#3410.

The original section that was used to configure a font has been moved to a configFont() function, and a new configExtensions() function uses it to set up the font extensions.

[codecov]

Codecov Report

❌ Patch coverage is 76.08696% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.71%. Comparing base (efcdd46) to head (e504edc).

Files with missing lines	Patch %	Lines
components/mjs/output/util.js	79.31%	6 Missing ⚠️
ts/output/common.ts	70.58%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1337      +/-   ##
===========================================
- Coverage    86.72%   86.71%   -0.01%     
===========================================
  Files          337      337              
  Lines        84145    84182      +37     
  Branches      4769     4773       +4     
===========================================
+ Hits         72971    72997      +26     
- Misses       11151    11162      +11     
  Partials        23       23              

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[dpvc]

I added a commit that includes support for generic font extensions that have dynamic font ranges, like mathjax-euler. It turns out that they reference the generic font, which was only meant as a placeholder for the actual font, but that wasn't being replaced. This commit fixes that, but unfortunately, it requires a static property on the output jax because the dynamic files don't have access to the output jax instance. So the typeset() method sets up that static value, so that any dynamic loads during the typesetting will go to the right font. We do it here in case there are several output jax instances with different fonts. We also set it during the output jax constructor, in case the dynamic files are loaded up front (like some of the documentation examples illustrate).

[zorkow]

lgtm.

[dpvc]

I just realized there is a potential problem, here. If there is a retry during the toDOM() call (for loading dynamic font ranges), then the genericFont will not be reset. So I need to made another commit to handle that.