Until now, the only food for thought on The Simpsons was whether Homer prefers glazed or jelly. But fans are in for a gastronomic treat next season when Paul McCartney and wife Linda weigh in on Lisa's decision to become a vegetarian. For the McCartneys, it's a chance ''to spread the vegetarian word to a wider audience,'' according to Mrs. McCartney. (The couple was assured that Lisa would remain a non-meat eater in subsequent episodes.) For producer David Mirkin, a self-proclaimed Beatles fanatic, it means all three surviving Beatles have now lent their voices to the show (Ringo Starr did it in 1991 and George Harrison in 1993), but he's not finished yet. Say Mirkin, ''You never know when we'll come up with an idea and say, 'Oh, this will be perfect for Julian.' ''