Tina Fey’s “Mean Girls” the musical is a triumphant burst of high-energy fun, artfully written and brilliantly executed.  This stage adaption is every bit as funny as the now-iconic 2004 film, and even more effective at delivering its message of self-confidence and kindness, thanks to good writing, great design, a boffo cast, and the guiding hand of director/choreographer Casey Nicholaw.  In a season of too few musicals, “Mean Girls” is a standout.