NEW YORK, Jan 8 (Reuters) - A forthcoming U.S. Supreme Court decision on President Donald Trump's use of emergency tariff powers could jolt financial markets, especially if the justices strike down the tariffs. The case, which could come as early as Friday, is set to determine whether Trump can invoke the International Emergency Economic Powers Act (IEEPA) to ​impose tariffs without congressional approval. During November arguments, the justices signaled skepticism about Trump’s authority to impose the ...