Add support for PHP 8.3 typed class constants

[lacatoire]

Summary

PHP 8.3 introduced native type declarations for class constants:

class Foo {
    const string BAR = 'baz';
    const int|string UNION = 42;
    const ?string NULLABLE = null;
}

This PR adds support for reading and exposing these types through the reflection API, following the same pattern already used by Property:

• Add Type|null $type parameter to Constant class constructor and getType() accessor
• Add getType() to ClassConstantIterator to read the type from the ClassConst AST node
• Pass the resolved type through in the ClassConstant factory via the existing Type helper

Global constants are not affected (PHP does not support typed global constants).