Added some missing obscure JSON Schema props.

[daveshanley]

I was reviewing the specs again, missed a couple of props I have not seen in the wild myself, but they should be included to be fully compliant.

$comment, contentSchema and $vocabulary

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (c3cf119) to head (c8fe000).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #495    +/-   ##
========================================
  Coverage   99.58%   99.58%            
========================================
  Files         189      189            
  Lines       22476    22649   +173     
========================================
+ Hits        22383    22556   +173     
  Misses         59       59            
  Partials       34       34            

Flag	Coverage Δ
unittests	99.58% <100.00%> (+<0.01%)	⬆️

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