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 86.72%. Comparing base (9ef060f) to head (97b2e4d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1532      +/-   ##
===========================================
+ Coverage    86.70%   86.72%   +0.01%     
===========================================
  Files          340      341       +1     
  Lines        86264    86378     +114     
  Branches      3246     3247       +1     
===========================================
+ Hits         74798    74913     +115     
+ Misses       11466    11465       -1     

☔ 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.