Support PHP 8.0 Union types RFC

[TysonAndre]

https://wiki.php.net/rfc/union_types_v2 was merged into PHP master today. The grammar is https://wiki.php.net/rfc/union_types_v2#type_grammar

Union types of 2 or more types can now occur on

• Properties
• Parameters
• Return types

For backwards compatibility of existing applications, this could be supported similarly to 59b37a8 - parse the first type in the union type (unless it started with ?, optionally), then parse any remaining types into a different property