BEIRUT — Islamic militants and rebels in Syria launched fresh attacks on government-held neighborhoods in Aleppo on Friday, setting off some of the heaviest fighting in months in the contested northern city, activists and state media said. The fighting is part of a new coordinated offensive in Aleppo by a newly formed coalition between al-Qaeda’s branch in Syria, the Nusra Front, and the ultra-conservative Ahrar al-Sham group, and other rebels.