More Combinatorial Game Theory Formalizations

Stream: mathlib4

[Tristan Figueroa-Reid]

As I mentioned in the GitHub Issue, I do believe it would be valuable to formalize more of combinatorial game theory past pre-games.

Being able to describe non-normal positional games, misère games, or loopy games would be nice: formalizing stoppers (or the 'end value' of a game) would help formalize the minimax algorithm and open up the way for formalizing aspects of combinatorial search theory related to combinatorial games.