Add mrgIfPropagatedStrategy

[lsrcz]

This pull request exposes the mrgIf function with a propagated merging strategy.

[codecov]

Codecov Report

Attention: Patch coverage is 31.57895% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 59.90%. Comparing base (e8962c3) to head (5bcddd9).

Files	Patch %	Lines
src/Grisette/Core/Data/Class/SimpleMergeable.hs	33.33%	16 Missing ⚠️
src/Grisette/Core/Data/Class/GenSym.hs	0.00%	9 Missing ⚠️
src/Grisette/Core/Data/Union.hs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   60.15%   59.90%   -0.25%     
==========================================
  Files          72       72              
  Lines        6149     6181      +32     
  Branches      505      508       +3     
==========================================
+ Hits         3699     3703       +4     
- Misses       1945     1970      +25     
- Partials      505      508       +3     

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