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

[masatake]

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

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.85%. Comparing base (b31106a) to head (82e721d).

Files with missing lines	Patch %	Lines
parsers/sh.c	76.92%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3544      +/-   ##
==========================================
- Coverage   85.85%   85.85%   -0.01%     
==========================================
  Files         244      244              
  Lines       62989    63026      +37     
==========================================
+ Hits        54082    54110      +28     
- Misses       8907     8916       +9     

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