input: Write a function to generate the first n Fibonacci numbers.
  assert generate_fibonacci(5) == [0, 1, 1, 2, 3]
  assert generate_fibonacci(10) == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
  assert generate_fibonacci(15) == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377]
output: def generate_fibonacci(n):
      if n <= 0:
          return []
      elif n == 1:
          return [0]
      fibonacci_numbers = [0, 1]
      while len(fibonacci_numbers) < n:
          fibonacci_numbers.append(fibonacci_numbers[-1] + fibonacci_numbers[-2])
      return fibonacci_numbers