Prevent moving CanGc values between threads/tasks (#33902)

* Make CanGc non-sendable, and add documentation.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Update CanGc usage to fix usages that were moved between threads/tasks.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/script_runtime.rs

@@ -1115,10 +1115,17 @@ impl Runnable {
 }
 
 #[derive(Clone, Copy, Debug)]
-pub struct CanGc(());
+/// A compile-time marker that there are operations that could trigger a JS garbage collection
+/// operation within the current stack frame. It is trivially copyable, so it should be passed
+/// as a function argument and reused when calling other functions whenever possible. Since it
+/// is only meaningful within the current stack frame, it is impossible to move it to a different
+/// thread or into a task that will execute asynchronously.
+pub struct CanGc(std::marker::PhantomData<*mut ()>);
 
 impl CanGc {
+    /// Create a new CanGc value, representing that a GC operation is possible within the
+    /// current stack frame.
     pub fn note() -> CanGc {
-        CanGc(())
+        CanGc(std::marker::PhantomData)
     }
 }