WASHINGTON — The Supreme Court said Monday it will decide whether to jettison a decades-old decision that has been a frequent target of conservatives and, if overruled, could make it harder to sustain governmental regulations.The justices agreed to hear an appeal that takes aim at a 1984 case Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837. It involves the Chevron oil company and says that when laws aren’t crystal clear, federal agencies should be allowed to fill in the details …