The US Federal Reserve has announced an $85 billion rescue package for AIG, the country's largest insurance company, to save it from bankruptcy and avoid plunging the financial markets into deeper crisis.   The rescue follows the collapse of Lehman Brothers, the fourth-largest investment bank in the US, in which the Fed did not intervene, causing share prices to plummet all over the world.    The Fed's move is viewed by some as the most radical intervention in private business in its history, and has helped fuel a tentative rally on global stock markets.