chore(algebra/group/units): is_add_unit.decidable (#18439)
Missing `to_additive`
I had actually added this to mathlib4 3 months ago in leanprover-community/mathlib4@cbc083419c0aeee4e99154f304f5f8ac66bce0d8 (leanprover-community/mathlib4#549), and never noticed it wasn't in mathlib3.
mathlib4 hash-update placeholder pr: https://github.com/leanprover-community/mathlib4/pull/2278
