perf: cache empty VList Rc in VNode::default to avoid repeated heap allocation

VNode::default() is called on every mem::take() during reconciliation.
Previously each call heap-allocated a new Rc<VList>. Use a thread_local
to cache a single Rc<VList> and reuse it via Rc::clone (refcount bump only).

Author: Madoshakalaka

packages/yew/src/virtual_dom/vnode.rs

@@ -106,7 +106,10 @@ impl VNode {
 
 impl Default for VNode {
     fn default() -> Self {
-        VNode::VList(Rc::new(VList::default()))
+        thread_local! {
+            static EMPTY_VLIST: Rc<VList> = Rc::new(VList::default());
+        }
+        VNode::VList(EMPTY_VLIST.with(Rc::clone))
     }
 }