Mini-rust in Rust 042: An unexpected token means stop this expression

We're on our way towards type-checking variables, but first we take another detour into parsing expressions: if we hit an unexpected token, we should backtrack and return what we found so far, not just stop with an error. More info: https://artificialworlds.net/blog/2025/08/15/mini-rust-in-rust-042-unexpected-token-means-stop-expression