A vibrant theater scene is one of the perks of living in the Boston area, and American Repertory Theater (A. R. T.) in Cambridge is often a great place to catch shows when they’re brand-new. Recently, I attended a performance of Wonder, a new musical adaptation of R. J. Palacio’s 2012 middle-grade novel (which, speaking of the Boston area, I remember flying off the shelves during my Brookline Booksmith bookseller days).