Add Diplomacy 🗺️

Implement the game of Diplomacy as a ZeroSumEval-compatible game. This game requires multi-agent negotiation, long-term planning, and coalition-building. A great fit for evaluating strategic communication and intent modeling in LLMs. Ideal for 3+ players. Includes challenges around turn structure and map-based game state.

This game is likely to be non-trivial to implement and play due to the large amount of information that needs to be encoded in text to be fed to an LLM. This could be a good candidate for multimodal models, where we could pass the current state of the board as an image instead.