In this challenge, you will implement a simple countdown timer using a while loop. The goal is to create a function that takes an integer n and returns a vector containing the countdown from n to 0.

Write a function countdown(n: u32) -> Vec<u32> that takes a non-negative integer n and returns a vector with the numbers from n to 0.

• Use a while loop to implement the countdown.
• Ensure the function handles the case where n is 0 correctly.
• The function should return a vector with each number in the countdown sequence.

let result = countdown(3);
assert_eq!(result, vec![3, 2, 1, 0]);
 
let result = countdown(0);
assert_eq!(result, vec![0]);