An add() method macro

[Gummibeer]

The $collection->add('key', 'value') will only add the given value if the key doesn't already exist on the collection.

$collection = collect([''foo' => 'bar']);

$collection->add('lorem', 'ipsum');
$collection->add('foo', 'foobar');

$collection->all() === [
  'foo' => 'bar',
  'lorem' => 'ipsum',
];

It would TBD if the add() method returns a new instance of the collection or if the already existing collection is manipulated.