json emitter: fixes to scalars with missing quotes

[biojppm]

fixes #291
fixes #292

[codecov]

Codecov Report

Merging #299 (fbec340) into master (f020b03) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #299   +/-   ##
=======================================
  Coverage   97.20%   97.20%           
=======================================
  Files          78       78           
  Lines       18052    18072   +20     
=======================================
+ Hits        17547    17567   +20     
  Misses        505      505           

Impacted Files	Coverage Δ
samples/quickstart.cpp	99.54% <100.00%> (+<0.01%)	⬆️
src/c4/yml/emit.def.hpp	92.19% <100.00%> (+0.04%)	⬆️
test/test_json.cpp	98.94% <100.00%> (+0.06%)	⬆️
test/test_suite/test_suite_events_emitter.cpp	100.00% <0.00%> (ø)

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