Implement readonly modifier for classes - #138

See https://wiki.php.net/rfc/readonly_classes and xp-framework/ast#37. The following comes quite close to records, although it doesn't declare getters but uses public readonly fields:

readonly class Point {
  public function __construct(
    public int $x,
    public int $y,
  ) { }
}

$point= new Point(8, 42);
echo "Hello {$point->x}x{$point->y}!\n";

// Will not modify the member, raising an exception instead:
// $point->x= 9;

• Inherit readonly modifiers on classes to all members and promoted constructor parameters
• Prevent dynamic members
• Do not allow readonly on traits, interfaces and enums
• Prevent inheritance where readonly modifiers do not match
• Prevent static members in readonly classes
• Disallow #[AllowDynamicProperties] on readonly classes
• Remove emulation for PHP 8.2 once Add support for readonly classes php/php-src#7305 is merged