Allow entity definitions to load synchronously, and add tests for it.

[dpvc]

This PR adds the ability to synchronously load entity extensions from node applications that have asyncIsSyncronous set. This is similar to the synchronous loading of dynamic font data font. A number of new tests are added, and the current single function is broken into several pieces.

It also removes a deprecated use of RegExp.$1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.93%. Comparing base (9ef060f) to head (cd5f2eb).
⚠️ Report is 139 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1532      +/-   ##
===========================================
+ Coverage    86.70%   86.93%   +0.22%     
===========================================
  Files          340      390      +50     
  Lines        86264    87833    +1569     
  Branches      3246     4984    +1738     
===========================================
+ Hits         74798    76355    +1557     
+ Misses       11466    11459       -7     
- Partials         0       19      +19     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[zorkow]

lgtm.

But I am wondering if the jsdoc in mathjax.ts

  /**
   * When asyncLoad uses require(), it actually operates synchronously and this is true
   */
  asyncIsSynchronous: false,

should be updated. asyncLoad could be a function using readFileSync in Node.

[dpvc]

asyncLoad could be a function using readFileSync in Node.

Only if the user replaces asyncLoad; of our ts/utility/asyncLoad/*.ts files, the only one that sets asyncIsSynchronous to true is node.ts, which uses require(). None of our versions use readFileSync().

But I will modify the comment to be more generic.