feat: add attributes to interfaces and traits

[calebdw]

Closes #251

I wasn't sure if attributes on interfaces is supported by PHP given that the docs state:

Classes, methods, functions, parameters, properties and class constants can be the target of an attribute.

However, the PHP parser does parse them:

https://github.com/php/php-src/blob/a7bd911ad6a6045a9619eb56814569bff1b8bd03/Zend/zend_language_parser.y#L384-L390

Other changes:

• added attributes to trait declarations
• fixed error when throw is used as a named argument
• fixed error when passing variable by reference to promoted property