[Chess] Rename turn to color

[sotetsuk]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.92%. Comparing base (c7274bd) to head (3a75aad).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pgx/_src/games/chess.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1248   +/-   ##
=======================================
  Coverage   79.92%   79.92%           
=======================================
  Files          61       61           
  Lines        5753     5753           
=======================================
  Hits         4598     4598           
  Misses       1155     1155           

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