Theorem finsupp.add_closure_Union_range_single

refactor(data/finsupp): use `{f : α →₀ M | ∃ a b, f = single a b}` instead of union of ranges (#10671)

feat(data/finsupp): lift a collection of `add_monoid_hom`s to an `add_monoid_hom` on `α →₀ β` (#4395)