feat(data/list/basic): list.mem_insert_nth (#1336)
feat(data/list/basic): list.mem_insert_nth (#1336)

* feat(data/list/basic): list.mem_insert_nth

* Update src/data/list/basic.lean

Co-Authored-By: Floris van Doorn <04eee3258902297ef5081ba8b0786f6ab1274795@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib