Mini-rust in Rust 023: Refactor to share structure of the syntax tree

We are repeating ourselves with the `SyntaxTree` and `TypedSyntaxTree` enums. I thought the solution would be too complex, but on reflection it's fine, so let's share that code. More info: https://artificialworlds.net/blog/2025/04/04/mini-rust-in-rust-023-share-syntax-tree-structure