Add support for `@?` operator

[togiolda]

Hi! The library that my project utilises (pg-mem) depends on psql-ast-parser library.

Recently I wanted to add support for @? operator for my pg-mem postgres instance. This errored and pointed me to this AST parser project. I checked the the source code of psql-ast-parser and actually couldn't find any definition of @? operator.

I'd be really grateful if you could have a look at this problem and consider adding the support for @? 🙌