input: Write a python function to generate a list of Fibonacci numbers less than a given number n.
  assert fibonacci(10) == [0, 1, 1, 2, 3, 5, 8]
  assert fibonacci(20) == [0, 1, 1, 2, 3, 5, 8, 13]
  assert fibonacci(1) == [0]
output: def fibonacci(n):
      fib_list = []
      a, b = 0, 1
      while a < n:
          fib_list.append(a)
          a, b = b, a + b
      return fib_list