Implement myFind(arr, callback)
Write myFind(arr, callback) that returns the first element for which callback(item, index, array) is truthy, or undefined if none match.
myFind([1, 4, 5, 8], (n) => n % 2 === 0); // 4
myFind([1, 3, 5], (n) => n % 2 === 0); // undefined