The subject matter of “Rabbit Hole,” David Lindsay-Abaire‘s quiet tearjerker about a family coping with the death of a 4-year-old, may not go hand-in-hand with partying hearty. But that didn’t stop Manhattan Theater Club from celebrating its latest Broadway offering on Thursday.Star Cynthia Nixon was enticed back to the stage by the melodrama-free script. “When subjects like this come up, plays are usually a little more Greek in their grieving,” she said. She and co-star John Slattery are parents of young children. “Doing this play is like a three-month therapy session,” Slattery said.