''I have nothing to declare,'' Wilde once told an American customs official, ''except my genius.'' A socialite, a wit, a man who flaunted convention and was unafraid to shock, Oscar Wilde was a great writer and a great man. This new collection of wit and wisdom demonstrates the brilliance of his vision, the audacity of his style. Such is the scope of the material, it brings to life the Wilde of great feeling as well as the Wilde of great art.