Burger King, whose quarter-pound Whopper pushed its competitors a half-century ago to create their own two-fisted hamburgers, now plans to roll out a vegetarian version of its signature sandwich, relying on plant-based patties developed by San Francisco Bay area start-up Impossible Foods. The Impossible Whopper will be introduced this week at Burger King restaurants in the St. Louis area — in the very state that last year banned the use of the term “meat” for any vegetarian or cell-based substitutes for animal-raised meats. No, this is not an April Fools’ Day joke. In fact, Burger King’s plan could be the...