At the movies, The Silence of the Lambs continues to slay 'em at the box office. In music, future Behind the Music star Rob Van Winkle (a.k.a. Vanilla Ice) dominates the album charts with To the Extreme. In bookstores, Danielle Steel and Dean Koontz top the fiction best-seller list. Her Heartbeat and his Cold Fireare Nos. 1 and 2, respectively. And in the news, Operation Desert Storm, the ground phase of which had been launched on Feb. 24, concludes with President Bush's declaration that ''Kuwait is liberated. Iraq's army is defeated.''