Add ISO 8601 string timestamp detection to fieldtypes

[JSCU-CNI]

This PR adds datetime fieldtype detection for ISO 8601 strings. Not sure what the performance implications of this change are in regard to fieldtype_for_value.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.25%. Comparing base (ff6a1f1) to head (0dc6ad1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
+ Coverage   84.24%   84.25%   +0.01%     
==========================================
  Files          35       35              
  Lines        3745     3748       +3     
==========================================
+ Hits         3155     3158       +3     
  Misses        590      590              

Flag	Coverage Δ
unittests	84.25% <100.00%> (+0.01%)	⬆️

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