mathlib3

0b9fbc48 - feat(combinatorics/simple_graph/basic): add definition of common neighbors and lemmas (#5718)

feat(combinatorics/simple_graph/basic): add definition of common neighbors and lemmas (#5718) Part of #5698 in order to prove facts about strongly regular graphs Co-authored-by: agusakov <39916842+agusakov@users.noreply.github.com>