The most-hyped line of movie-themed merchandise in history — for “Star Wars: Episode I — The Phantom Menace,” which opens May 19 — finally went on sale at midnight today. In an unprecedented move, Toys R Us stores around the country decided to open their doors at 12:01 a.m. Monday for an all-night selling marathon of toys based on the movie. FAO Schwarz on New York’s Grand Army Plaza opened from 12 to 2 a.m. to accommodate 1,500 fans who had begun to line up along Fifth Ave. the previous afternoon. Wal-Mart and Kmart called in extra staff for their 24-hour stores to handle their post-midnight crowds. Many stores limited the number of items a customer could buy to prevent immediate sell-outs.