countBy(['apple', 'banana', 'avocado'], s => s[0])
// → { a: 2, b: 1 }countBy counts array elements by a derived key, producing a Record of occurrence counts.
Challenge: Implement countBy<T>(arr: T[], fn: (x: T) => string): Record<string, number> counting items grouped by a computed key.
countBy(['a', 'b', 'aa', 'bb', 'aaa'], s => String(s.length))
// → { '1': 2, '2': 2, '3': 1 }