About three years ago, playwright David Henry Hwang was walking near his home in Brooklyn when suddenly he was stabbed in the neck by an unknown assailant. Hwang suffered a pretty serious wound, requiring surgery and several days of recovery in the hospital. So what does he do afterwards? He riffs on that experience by writing a play, of course. And not just a play, but a gleeful musical comedy on which he collaborated with composer Jeanine Tesori (they share lyric writing credits). The result of their efforts is Soft Power, opening tonight at the Public Theater, and it is a merry delight. . . .