Add builtin array methods

[Folyd]

let numbers = [10, 5, 8, 3, 1];
print(numbers);  // expect: [10, 5, 8, 3, 1]

// append - Add an item to the end
numbers.append(20);
print(numbers);  // expect: [10, 5, 8, 3, 1, 20]

// extend - Extend with another array
numbers.extend([30, 40]);
print(numbers);  // expect: [10, 5, 8, 3, 1, 20, 30, 40]

// insert - Insert an item at position
numbers.insert(2, 15);
print(numbers);  // expect: [10, 5, 15, 8, 3, 1, 20, 30, 40]

// index - Find position of value
let pos = numbers.index(15);
print(pos);  // expect: 2