Check for a purely emoji string and PHP 8.1 Support

This release adds a feature where you can check if a string is purely emojis. For example:

<?php

require_once('vendor/autoload.php')

use SteppingHat\EmojiDetector;

$detector = new EmojiDetector();

$detector->isEmojiString("😂😂😂"); // Returns TRUE
$detector->isEmojiString("Deez nuts 🥜") // Returns FALSE

This update also brings automated testing for PHP 8.1