Implement FLOW_ML style

[biojppm]

Both for emitting and detection while parsing

[codecov]

Codecov Report

❌ Patch coverage is 99.49341% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.58%. Comparing base (d820acb) to head (f8ee5fb).
⚠️ Report is 68 commits behind head on master.

Files with missing lines	Patch %	Lines
src/c4/yml/emit.def.hpp	99.37%	3 Missing ⚠️
src/c4/yml/parse_engine.def.hpp	98.78%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #550      +/-   ##
==========================================
+ Coverage   97.45%   97.58%   +0.13%     
==========================================
  Files          46       46              
  Lines       13432    13641     +209     
==========================================
+ Hits        13090    13312     +222     
+ Misses        342      329      -13     

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