DAVID DEL TREDICI will be composer-in-residence at the New York Philharmonic during its 1989-90 and '90-91 seasons. The California Pulitzer winner's activities will include writing a work to be premi`ered next season, advising on programming, and planning for a new-music series. THE NEW YORK FILM FESTIVAL is in full swing this week after its Friday opening. This 26th edition, which continues through Oct. 9, features more works by women directors than ever before, and special focuses on new British films, Soviet cinema in the age of glasnost, new Asian films, and American independents. More than 400 films were screened by the festival's selection committee, whose new chairman is Richard Pena, a director of the Chicago Film Center and an outspoken advocate of third-world cinema. JOHN DENVER will appear on his first Country Music Association Awards Show Oct. 10. Denver, whose hits include ``Thank God I'm a Country Boy,'' will present the 1988 Song of the Year Award during the nationally televised show. PEPSICO SUMMERFARE IS CALLING IT QUITS after the 1989 season. The eight-year-old Purchase, N.Y., music festival has been known for its innovative opera, dance, and theater. A joint announcement last week by Christopher Hunt, artistic director since 1985, and Sheldon Grebstein, president of the State University of New York at Purchase, the festival's presenter, said the financial burden for the university has become too great to continue. Corporate funding comes from PepsiCo. Next summer's program will go on as planned.