--posix should reject GNU substitute flags i/I #401

[Devel08]

resolves #401

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.15%. Comparing base (f783eb9) to head (9e61362).

Files with missing lines	Patch %	Lines
src/sed/compiler.rs	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
- Coverage   82.20%   82.15%   -0.06%     
==========================================
  Files          13       13              
  Lines        5542     5547       +5     
  Branches      310      312       +2     
==========================================
+ Hits         4556     4557       +1     
- Misses        983      987       +4     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	82.83% <77.77%> (-0.06%)	⬇️
ubuntu_latest	82.94% <77.77%> (-0.06%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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