OCCASIONAL LISTENERS to ''Extreme Games" on WTKK are accustomed to host Jay Severin admiring himself in the mirror of his own imagination, a glass so vast and glittering as to rival the Hall of Mirrors at Versailles. But last Friday, the talkmaster positively outdid himself in setting new laurels upon his brow: He awarded himself a Pulitzer Prize. That came as part of a conversation with a caller about the declining standards he sees in journalism. Here's what Jay said: ''But since journalism began, and up until the time at least that I took my master's degree at Boston University...