feat(algebra/group_power/basic): add abs_add_eq_add_abs_iff (#6593)
feat(algebra/group_power/basic): add abs_add_eq_add_abs_iff (#6593)

I've added
```
lemma abs_add_eq_add_abs_iff {α : Type*} [linear_ordered_add_comm_group α]  (a b : α) :
  abs (a + b) = abs a + abs b ↔ (0 ≤ a ∧ 0 ≤ b ∨ a ≤ 0 ∧ b ≤ 0)
```
from `lean-liquid`. For some reasons I am not able to use `wlog hle : a ≤ b using [a b, b a]` at the beginning of the proof, Lean says `unknown identifier 'wlog'` and if I try to import `tactic.wlog` I have tons of errors.



leanprover-community/mathlib,leanprover-community/mathlib