Bucks County detectives are investigating the apparent suicide of an Upper Southampton police officer.   55-year-old Richard Lizzio was found in his patrol vehicle Friday morning dead from a shotgun wound outside a church near Bristol Road.   Landscapers made the grisly discovery.   The police chief from neighboring Ivyland suffered a heart attack while responding to the scene.   Chief Nick Rosato is in stable condition at St. Mary Medical Center in Langhorne.