feat(data/finsupp/to_dfinsupp): add sigma_finsupp_lequiv_dfinsupp (#7818)
feat(data/finsupp/to_dfinsupp): add sigma_finsupp_lequiv_dfinsupp (#7818)

Equivalences between `(Σ i, η i) →₀ N` and `Π₀ i, (η i →₀ N)`.

- [x] depends on: #7819 



Co-authored-by: Eric Wieser <da64e90baec5d084ce3d8e05f3b022ae6d118ff5@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib