feat(order/directed): a subset stable by supremum is `directed_on (≤)` (#16054)
feat(order/directed): a subset stable by supremum is `directed_on (≤)` (#16054)




Co-authored-by: Eric Wieser <da64e90baec5d084ce3d8e05f3b022ae6d118ff5@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib