This extends the LibJSGC Clang plugin to detect GC pointers (GC::Ptr,
GC::Ref, JS::Value, etc.) inside non-GC-allocated struct/class members.
When a GC::Cell has a member of a non-Cell type that contains GC
pointers, we now enforce that:
1. The non-Cell type must have a visit_edges(GC::Cell::Visitor&) method
2. The Cell's visit_edges must access that member (presumably to call
its visit_edges)
The check works recursively, so nested structs and containers like
Vector<GC::Ptr<T>> or HashMap<K, GC::Ptr<V>> are handled correctly.
GC infrastructure types (Root, Heap, etc.) and AK library types are
excluded from these checks as they handle visitation differently.
