feat: prove the Ramsey theorem for infinite graphs #281
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This PR proves the Ramsey theorem for infinite graphs, which will be needed in Buchi's proof that omega-regular languages are closed under complementation.
Ideally, this result should be in mathlib, but it is not and I don't know when it will be:
https://leanprover.zulipchat.com/#narrow/channel/287929-mathlib4/topic/Infinite.20Ramsey.20theory/with/568507228
So I now prove what I will need. Once mathlib has the desired result, this proof can be deprecated.