add support for traceable collections to Rust

[anonrig]

Addresses the following comments by @jasnell:

• add gc support to Rust owned objects #6272 (comment)
• add gc support to Rust owned objects #6272 (comment)
• add gc support to Rust owned objects #6272 (comment)

[ask-bonk]

LGTM

github run

[guybedford]

I'm sorry I just don't think this is the right approach - matching name strings is clearly in violation of scoping rules, as I've mentioned before.

Please explain to me why we aren't using a trait to handle this?

[anonrig]

Addressed the review feedback:

• Reworked GC tracing to be trait-driven via Traced instead of field-type string matching.
• GarbageCollected now extends Traced; trace() lives on Traced.
• Removed #[jsg_traceable] / #[jsg_trace] and updated resource tracing to call Traced::trace() on every field.
• Added no-op Traced impls for non-traceable/value types and delegating impls for wrappers/collections.
• Updated docs/comments (including jsg_struct wording around class_name() usage and macro table formatting).
• just clippy passes on this branch.

Could you please take another look?

[jasnell]

Findings from review:

• [MEDIUM] name + custom_trace combination silently drops the name override — extract_name_attribute parses the full attr stream as a single MetaNameValue, which fails when custom_trace is also present
• [MEDIUM] has_custom_trace_flag uses string-based parsing rather than syn meta parsing, which is inconsistent with the rest of the crate
• [LOW] RefCell<T> tracing uses self.borrow() which will panic if a mutable borrow is active during GC

This review was generated by an AI assistant and may contain inaccuracies.

[guybedford]

Please can we just add one more thing to this PR - a derive macro for Traced on structs and enums?

Example:

#[derive(jsg::Traced)]
struct MyPlainData { ... }

#[derive(jsg::Traced)]
enum MyPlainData { ... }

basically works just like the current trace.rs does anyway.

[guybedford]

Marking this as conditional approval on the comments and derive macro being added.