Respect namespace style for enum scalar type printing

Doesn't matter in any practical sense because such types are always
invalid, but makes sure pretty printing round-trips.

Remove PHP 5 parser

Add handling for Enum(Case)s in NameResolver

[PHP 8.1] Add support for enums (#758)

RFC: https://wiki.php.net/rfc/enumerations

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>