Good news for people who grew up delighting in the weirdo charms of Marty McFly and the DeLorean: The new, two-and-a-half-hour “Back to the Future” musical — which officially opened on Broadway Thursday night — is an incredibly faithful retelling of the 1985 movie, down to classic lines (“My dad’s a peeping Tom!”) and awkward as hell relationship drama. As for everyone else?