enhance: make the argument of `#[serde(validate = "...")]` correctly recognized by rust-analyzer

[kanarus]

Rust codes included in serde's helper attributes like #[serde(default = "default_fn")] or #[serde(skip_serialize_if = "Option::is_none")] are, despite of being string, correctly recognized as Rust codes by rust-analyzer, with

goto definition, syntax highlight, documentation, and much more...

features, just except for syntax highlight. (but in my memory, VSCode could even apply it...)

But #[serde(validate = "...")] is not recognized, maybe due to unsound proc-macro implementation.

[kanarus]

Also, handle span correctly: Currently, any issues on the validate = "..." argument is reported on Deserialize span.