Add Checkers 🔴⚫

Implement the game of Checkers in ZeroSumEval. This classic two-player board game focuses on spatial reasoning, forced moves, and multi-step planning. Should support standard American rules, including capturing and king promotion. Useful for benchmarking strategic foresight in simpler game environments than Chess.

The structure of the implementation would likely look similar to the current Chess implementation.

Start by reading the current Chess implementation and identify the game structure that a new game must follow. Implement standard American Checkers with capturing, forced moves, multi-step planning, and king promotion. Done means Checkers is available in ZeroSumEval and supports the stated rules.

Written by the indexing model from the issue text.