In deciding whether federal drug labeling law preempted the failure-to-warn claims asserted by musician who developed gangrene and ultimately needed to have her forearm amputated as alleged result of drug manufacturer's failure to adequately warn of dangers posed by intravenous administration of drug, court would not defer to statement made by the FDA in preamble to regulation, that the Federal Food, Drug, and Cosmetic Act establishes "both a 'floor' and a 'ceiling' " such that "FDA approval of labeling preempts conflicting or contrary State law," where the FDA had finalized regulation without offering states or other interested parties notice or opportunity for comment, and wherestatements made in preamble reversed the FDA's own longstanding position without providing any reasoned explanation, including any discussion of how state law had interfered with the FDA's regulation of drug labeling during decades of coexistence.