Chrysler plans to halt production at all 30 of its factories for one month, and is reportedly revisiting talks of a merger with General Motors, as both firms are witnessing their cash reserves dwindle.   The moves come as other US and foreign automakers are announcing steep production cuts that will idle tens of thousands of other US workers as the industry copes with withered demand for new cars and trucks. Ford said yesterday that it would stop production for an extra week in January at all but two of its plants because of flagging consumer demand.