Add basic filter for PHP 8 attribute at class level

[MarkCenturion]

Filter classes based on presence of a PHP 8 attribute on a class level

Note this is a basic implementation that simply looks for at least one declaration of a given attribute class and considers this a match. Uses IS_INSTANCEOF by default thus automatically handling inheritance in attribute classes.

Attribute class name must be passed, not an instance of the attribute.

#[Attribute]
class CoolStuff { ... }

#[CoolStuff]
class BeCool { ... }

// Print all classes tagged with a CoolStuff attribute
foreach ($iter->attribute(CoolStuff::class) as $class) {
echo $class->getName();
}

Note this works only in PHP 8.0.1 or higher, but syntax is PHP 7 compatible so whilst this cannot be used on earlier versions it also won't break them. Tests should pass in PHP 8 or higher and be skipped otherwise.