feat(order/countable_dense_linear_order): Relax conditions of `embedding_from_countable_to_dense` (#12928)
feat(order/countable_dense_linear_order): Relax conditions of `embedding_from_countable_to_dense` (#12928)

We prove that any countable order embeds in any nontrivial dense order. We also slightly golf the rest of the file.





leanprover-community/mathlib,leanprover-community/mathlib