feat: add ExtendedClassAwareRuleInterface to detect extended classes

[samsonasik]

This PR introduces Boundwize\StructArmed\Rule\ExtendedClassAwareRuleInterface.

A marker that lets a rule know whether the class it's evaluating is extended by another scanned class, and applies it to Boundwize\StructArmed\Rule\Rules\Class_\MustBeFinalRule.

Using interface in custom rule:

use Boundwize\StructArmed\Analyser\ClassNode;
use Boundwize\StructArmed\Rule\ExtendedClassAwareRuleInterface;
use Boundwize\StructArmed\Rule\RuleViolation;

final readonly class MyRule implements ExtendedClassAwareRuleInterface
{
    public function appliesTo(ClassNode $classNode): bool
    {
        return $classNode->isClass();
    }

    public function evaluate(ClassNode $classNode): ?RuleViolation
    {
        // Skip classes that another scanned class extends.
        if ($classNode->isExtended) {
            return null;
        }

        // other check ...

        return new RuleViolation(
            ...
        );
    }
}

[codecov]

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