input: Write a function to create a Python class called Rectangle that represents a two-dimensional rectangle with a length and width. The class should have an __init__ method to initialize the length and width, and an area method to calculate and return the area of the rectangle. If the length or width is negative, the class should raise a ValueError.
  assert Rectangle(5, 4).area()==20
  assert Rectangle(10, 10).area()==100
  try:
      Rectangle(-5, 4)
  except ValueError:
      assert True
  else:
      assert False
  try:
      Rectangle(5, -4)
  except ValueError:
      assert True
  else:
      assert False
output: class Rectangle:
      def __init__(self, length, width):
          if length < 0 or width < 0:
              raise ValueError("Length and width must be non-negative")
          self.length = length
          self.width = width
  
      def area(self):
          return self.length * self.width