Barbie she's not. Meet Gwen Thompson, the newest addition to the American Girl canon of dolls -- the wildly suc cessful, extremely expensive brand of faux children that are sold out of a four-story town house in the heart of Fifth Avenue. Little children as young as 4 are addicted to these pricey little monsters. It's like middle-American crack. You have an African-American doll, an American Indian doll. A Jewish one. A doll who "lived" during the Great Depression, and one from the Roaring '20s. And while you were snoozing, the creators of American Girl, which is sold by Mattel,...