[Shogi] Make core singlefile

[sotetsuk]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 98.83721% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.28%. Comparing base (1e08b48) to head (f1c2dbc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pgx/_src/games/shogi.py	98.79%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1273      +/-   ##
==========================================
- Coverage   80.29%   80.28%   -0.01%     
==========================================
  Files          62       62              
  Lines        5881     5879       -2     
==========================================
- Hits         4722     4720       -2     
  Misses       1159     1159              

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