WASHINGTON — Hunter Biden was indicted Thursday on federal firearms charges, the latest and weightiest step yet in a long-running investigation into the president’s son.Biden is accused of lying about his drug use when he bought a firearm in October 2018, a period when he has acknowledged struggling with addiction to crack cocaine, according to the indictment filed in federal court in Delaware by a special counsel overseeing the case.President Joe Biden’s son has also been under investigation for his …