Anne Anka, who's been married to the singer ("Diana") and songwriter ("My Way") Paul Anka, 59, since 1963, has filed for legal separation on grounds of irreconcilable differences, according to papers filed yesterday in Los Angeles Superior Court. She is asking for an unspecified amount of spousal support. In a statement, Paul Bloch, publicist for the '50s teen idol, called the split "very amicable . . . They remain the best of friends, and they're all going to be together on Thanksgiving."