Use retryAfter() and handleRetriesFor() from mathjax so they are easier to override

[dpvc]

This PR moves the references to retryAfter() and handleRetriesFor() to the version in the mathjax variable rather than from the Retries.ts module directly. This makes it easier to override them. I have an example of making MathJax load tex extensions synchronously that would be helped by this, as that means there would be fewer methods that need to be overridden. Together with the synchronous loading of dynamic font data and of entity definitions, this would allow MathJax to be run synchronously in node applications, where synchronous file loading can be achieved.

Note that asyncLoad uses mathjax.asyncLoad internally, so we don't have to use mathjax for that.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.92%. Comparing base (6c2be35) to head (75043d5).

Files with missing lines	Patch %	Lines
ts/core/MathDocument.ts	0.00%	3 Missing ⚠️
ts/output/common/FontData.ts	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1547      +/-   ##
===========================================
- Coverage    86.92%   86.92%   -0.01%     
===========================================
  Files          390      390              
  Lines        87853    87852       -1     
  Branches      4984     4984              
===========================================
- Hits         76370    76369       -1     
  Misses       11464    11464              
  Partials        19       19              

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[zorkow]

I have two concerns about the new async loading. Please have a look.

[zorkow]

This now returns a promise regardless of if it fails or not. So the try...catch does not really have an effect on this. But that means that dynamic.failed is never set. That would mean the file is set to loaded
even if the promise failed.

[zorkow]

Similarly, should the promise failure here not be handled explicitly?