0.22.1: parser: avoid hitting prettyplease unimplemented path

[MathisWellmann]

by rejecting Item::Verbatim

For example, this code previously hit the unimplemented path in prettyplease, tearing down the entire run:

pub fn action(state: &mut usize) {
    *state += 1;
}
const SELL_ID: User200;

This is now rejected and backpressure is applied to the LLM.