Change reorder function to take values instead of Item

[khalyomede]

To make the API easier, we should be able to pass the value directly instead of saving an instance of the item.

This

use Khalyomede\ReorderBeforeAfter\Listing;
use Khalyomede\ReorderBeforeAfter\Placement;

$bag = new Item("bag", 1);
$book = new Item("book, 2);

$listing = new Listing([$bag, $book]);

$listing->reorder($bag, Placement::After, $book);

Would become this

use Khalyomede\ReorderBeforeAfter\Listing;
use Khalyomede\ReorderBeforeAfter\Placement;

$listing = new Listing([new Item("bag", 1), new Item("book", 2)]);

$listing->reorder("bag", Placement::After, "book");