Implement a function groupBy(items, key) that groups an array of objects by a property and 
returns an object where keys are group identifiers and values are arrays of the grouped items.

Note:
The key will always be a string.
Do not mutate the original array.

function groupBy(items, key) {}

// Example
const products = [
  { id: 101, category: 'electronics', name: 'Laptop', price: 1200 },
  { id: 102, category: 'electronics', name: 'Smartphone', price: 800 },
  { id: 103, category: 'furniture', name: 'Chair', price: 150 },
  { id: 104, category: 'furniture', name: 'Table', price: 300 },
  { id: 105, category: 'clothing', name: 'T-Shirt', price: 20 },
  { id: 106, category: 'clothing', name: 'Jeans', price: 40 }
];

// Usage
console.log(groupBy(products, 'category'));

// {
//   electronics: [
//     { id: 101, category: 'electronics', name: 'Laptop', price: 1200 },
//     { id: 102, category: 'electronics', name: 'Smartphone', price: 800 }
//   ],
//   furniture: [
//     { id: 103, category: 'furniture', name: 'Chair', price: 150 },
//     { id: 104, category: 'furniture', name: 'Table', price: 300 }
//   ],
//   clothing: [
//     { id: 105, category: 'clothing', name: 'T-Shirt', price: 20 },
//     { id: 106, category: 'clothing', name: 'Jeans', price: 40 }
//   ]
// }