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 28.57143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.98%. Comparing base (6c2be35) to head (f9610c3).
⚠️ Report is 41 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/core/MathDocument.ts	0.00%	3 Missing ⚠️
ts/output/common/FontData.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1547      +/-   ##
===========================================
+ Coverage    86.92%   86.98%   +0.05%     
===========================================
  Files          390      390              
  Lines        87853    87764      -89     
  Branches      4984     4983       -1     
===========================================
- Hits         76370    76343      -27     
+ Misses       11464    11402      -62     
  Partials        19       19              

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[zorkow]

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

[dpvc]

I have put back the mathjax.asyncLoad() calls to what they used to be. Do you need to re-review?

[zorkow]

I have put back the mathjax.asyncLoad() calls to what they used to be. Do you need to re-review?

No, that looks good now.