Add AsHandleValue trait to Heap<Value> and make Heap values rooted (#38024)

Encapsulates the unsafe conversion from Heap<Value> to HandleValue<'a>,
and reducing repetitive unsafe code at call.

fix #37258

components/script/dom/element.rs

@@ -422,6 +422,7 @@ impl Element {
         self.ensure_rare_data().custom_element_definition = None;
     }
 
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
     pub(crate) fn push_callback_reaction(&self, function: Rc<Function>, args: Box<[Heap<JSVal>]>) {
         self.ensure_rare_data()
             .custom_element_reaction_queue