input: Write a python function to generate the first n Fibonacci numbers and return them as a list.
  assert fibonacci_series(6) == [0, 1, 1, 2, 3, 5]
  assert fibonacci_series(8) == [0, 1, 1, 2, 3, 5, 8, 13]
  assert fibonacci_series(10) == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
output: def fibonacci_series(n):
      if n <= 0:
          return []
      elif n == 1:
          return [0]
      elif n == 2:
          return [0, 1]
      
      fibs = [0, 1]
      while len(fibs) < n:
          fibs.append(fibs[-1] + fibs[-2])
      return fibs