Nat.lt_add_left

Stream: mathlib4

[Anatole Dedecker]

In [Std/Data/Nat/Lemmas]https://github.com/leanprover/std4/blob/2919713bde15d55e3ea3625a03546531283bcb54/Std/Data/Nat/Lemmas.lean#L74-L75 we have docs4#Nat.lt_add_right but there is no analog of docs#nat.lt_add_left :sweat_smile:
I'm going to add it in Data/Nat/Basic for now, but we probably want this to be in Std too @Mario Carneiro

[Mario Carneiro]

I'll do another pass over Data.Nat.{Defs, Basic} later to port everything to std so either way is fine