Add a basic schema validation for config files

[kquinsland]

Good for basic validation.
More work (and probably some CI automation?) would be needed for precisely validating each of the possible custom checks.

This should resolve: #899

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.66%. Comparing base (dbd7529) to head (18bb9aa).
Report is 133 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #978       +/-   ##
===========================================
- Coverage   62.36%   28.66%   -33.70%     
===========================================
  Files         197      209       +12     
  Lines        4854     5947     +1093     
===========================================
- Hits         3027     1705     -1322     
- Misses       1439     4126     +2687     
+ Partials      388      116      -272     

Flag	Coverage Δ
unit	28.66% <ø> (-33.70%)	⬇️

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