feat(category_theory/limits): split coequalizers (#5230)
Define what it means for a triple of morphisms `f g : X ⟶ Y`, `π : Y ⟶ Z` to be a split coequalizer, and show that every split coequalizer is a coequalizer and absolute.
Define split pairs and `G`-split pairs.
These definitions and constructions are useful in particular for the monadicity theorems.
