feat(category_theory/abelian): biproducts of projective objects are projective (#7319)
feat(category_theory/abelian): biproducts of projective objects are projective (#7319)

Also all objects of `Type` are projective.





Co-authored-by: Scott Morrison <63f04b4d96235eb042cc46d94f14d241d2a26722@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib