Promise.finally

Implement your own version of Promise.prototype.finally.

Requirements:

• Add a finally method to Promise prototype
• Execute callback regardless of promise outcome (resolve or reject)
• Don't modify the resolved/rejected value
• The callback should not receive any arguments

Example:

Promise.resolve('success')
  .finally(() => console.log('Cleanup'))
  .then(value => console.log(value)); // 'success'

Promise.reject('error')
  .finally(() => console.log('Cleanup'))
  .catch(err => console.log(err)); // 'error'