For the most part … Supergirl is just another young, alienated extraterrestrial grappling with some eternal existential hurdles, some seriously far out. (Warner Bros. Pictures) By MANOHLA DARGIS A few times early in “Supergirl,” its heroine looks like a mess — a beautifully styled and scrupulously photographed one, true, but a wreck nevertheless. She likes to party hard, this chick, and will slam down drinks as heedlessly as any bro on a bender before crawling into bed alone on her artfully…
