Code Review

This pull request introduces serde serialization and deserialization for the IDL AST, which is a great feature for interoperability. The implementation is mostly solid, using conditional compilation to make serde support optional.

I've left a few comments regarding the serde attributes:

• There's a confusing and likely incorrect use of #[serde(untagged)] on an enum variant, which should be refactored for clarity and correctness.
• There are some inconsistencies in handling empty Vec fields during serialization. Applying skip_serializing_if = "Vec::is_empty" uniformly would make the JSON output cleaner and more consistent.

Overall, this is a valuable addition. Addressing these points will improve the maintainability and robustness of the code.