WASHINGTON — Senator Elizabeth Warren vowed on Friday to pass major health care legislation in her first 100 days as president, unveiling a new, detailed plan to significantly expand public health insurance coverage as a first step, and promising to pass a “Medicare for all” system by the end of her third year in office that would cover all Americans. The initial bill she would seek to pass if elected would be a step short of the broader Medicare for all plan she has championed. But it would substantially expand the reach and generosity of public health insurance, creating a...