[WIP] Sh: [EXPERIMENTAL] extract variables in variable assignments

[masatake]

This change extracts X in "X=...".

[codecov]

Codecov Report

❌ Patch coverage is 73.17073% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.03%. Comparing base (72c69ba) to head (aa57b23).

Files with missing lines	Patch %	Lines
parsers/sh.c	73.17%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3544      +/-   ##
==========================================
- Coverage   86.04%   86.03%   -0.01%     
==========================================
  Files         254      254              
  Lines       63762    63801      +39     
==========================================
+ Hits        54862    54890      +28     
- Misses       8900     8911      +11     

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