Load font files synchronously, when possible. (#1458)

[dpvc]

This PR allows font ranges to be loaded synchronously if the mathjax.asyncLoad() method is marked as synchronous. That makes it possible to use the synchronous typesetting and conversion calls in more situations.

Resolves issue #1458.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.46%. Comparing base (a65456c) to head (e33f654).

Files with missing lines	Patch %	Lines
ts/output/common/FontData.ts	0.00%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1500      +/-   ##
===========================================
- Coverage    86.47%   86.46%   -0.02%     
===========================================
  Files          340      340              
  Lines        86125    86135      +10     
  Branches      3206     3206              
===========================================
  Hits         74476    74476              
- Misses       11649    11659      +10     

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