Support union types

[j6s]

While reviewing #14 we realized that union types (such as Foo|Bar) are not supported yet.

• Union types in docblock typehints should be parsed as each of the component
• Union types in actual type declarations ( https://wiki.php.net/rfc/union_types_v2 ) should be parsed
• Union types in generics (e.g. Baz<Foo|Bar>) should also be parsed as each component + the wrapper

[bestform]

Keep in mind that this feature will be added in PHP8. As this project requires PHP 7.2 or higher, it either needs to be bumped to PHP8 or the code must reflect this by skipping those cases or it will probably break when running with PHP7 - at least when running the tests while trying to parse the respective test classes.

[j6s]

That I hadn't considered, yeah. For the beginning just supporting it in docblocks would go a long way.