Also allow using defaults from `JSON3.read`

[kshyatt]

That method bypasses constructfrom, so we need this as well to directly read in instances with missing fields that have defaults.

[codecov[bot]]

Codecov Report

Merging #80 (61f9770) into main (aaa7b87) will decrease coverage by 0.39%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   72.70%   72.31%   -0.40%     
==========================================
  Files           2        2              
  Lines         370      372       +2     
==========================================
  Hits          269      269              
- Misses        101      103       +2     

Impacted Files	Coverage Δ
src/StructTypes.jl	71.74% <0.00%> (-0.40%)	:arrow_down:

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