Only a boy preacher who had grown up to become one of America's most eminent writers could have produced a play like The Amen Corner. For to his first work for the theater James Baldwin brought all the fervor and majestic rhetoric of the storefront churches of his childhood along with an unwavering awareness of the price those churches exacted from their worshipers.   For years Sister Margaret Alexander has moved her Harlem congregation with a mixture of personal charisma and ferocious piety.  But when Margaret's estranged husband, a scapegrace jazz musician, comes home to die, she is in danger of losing both her standing in the church and the son she has tried to keep on the godly path. The Amen Corner is a play about faith and family, about the gulf between black men and black women and black fathers and black sons. It is a scalding, uplifting, sorrowful and exultant masterpiece of the modern American theater.