mean 
Calculates the average of an array of numbers.
If the array is empty, this function returns NaN.
Signature 
typescript
function mean(nums: number[]): number;Parameters 
- nums(- number[]): An array of numbers to calculate the average.
Returns 
(number): The average of all the numbers in the array.
Examples 
typescript
const numbers = [1, 2, 3, 4, 5];
const result = mean(numbers);
// result will be 3
