Add Sniff to ensure usage of Stringable interface

[jaymcp]

Description

Introduces a new Sniff that will flag when a class that defines the __toString magic method doesn't implement the Stringable interface.

This Sniff includes a fixer.

Testing Steps

Install this PR branch:

composer require --dev bigbite/phpcs-config:dev-feat/stringable

Create a file called class-my-class.php:

<?php

class My_Class {
	public function __toString(): string {
		return __CLASS__;
	}
}

Run PHPCS:

./vendor/bin/phpcs -ps --standard=BigBite class-my-class.php

If on PHP > 8.0:
Amongst the errors should be one labelled BigBite.Classes.Stringable, which should inform you that the class should implement the Stringable interface.

If on PHP < 8.0:
There should not be an error labelled BigBite.Classes.Stringable amongst the list of errors, since the Stringable interface was only introduced in PHP 8.0.

Types of changes (if applicable):

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist (if applicable):

• All checks pass when running composer run all-checks.