input: Write a python function to add two 2D vectors and return a string representation of the result.
  assert str(Vector2D(3, 4) + Vector2D(1, 2)) == '<4, 6>'
  assert str(Vector2D(0, 0) + Vector2D(0, 0)) == '<0, 0>'
  assert str(Vector2D(-1, -2) + Vector2D(3, 4)) == '<2, 2>'
output: class Vector2D:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
      def __add__(self, other):
          return Vector2D(self.x + other.x, self.y + other.y)
  
      def __str__(self):
          return f'<{self.x}, {self.y}>'