feat(measure_theory/measurable_space): add `prod.measurable_singleton_class` (#16559)
feat(measure_theory/measurable_space): add `prod.measurable_singleton_class` (#16559)

* golf `pi.measurable_singleton_class`, rename from `measurable_singleton_class.pi`;
* add `prod.measurable_singleton_class`.





leanprover-community/mathlib,leanprover-community/mathlib