SHE'S DONE THE NUN thing twice, and allowed Patrick Swayze's ghost to inhabit her body while making love to Demi Moore on screen. So there was only one challenge left for Whoopi Goldberg, and last week she accepted it. Come February, she'll replace Nathan Lane as the wily Roman slave in the Broadway revival of ""A Funny Thing Happened on the Way to the Forum.'' The producers haven't decided yet whether she'll play the role as a man or a woman.