refactor(reactivity): use more efficient reactive checks

WeakSets and WeakMaps shows degrading performance as the amount of
observed objects increases. Using hidden keys result in better
performance especially when repeatedly creating large amounts of
reactive proxies.

This also makes it possible to more efficiently declare non-reactive
objects in userland.

packages/runtime-core/src/vnode.ts

@@ -103,7 +103,14 @@ export type VNodeNormalizedChildren =
   | null
 
 export interface VNode<HostNode = RendererNode, HostElement = RendererElement> {
-  _isVNode: true
+  /**
+   * @internal
+   */
+  __v_isVNode: true
+  /**
+   * @internal
+   */
+  __v_skip: true
   type: VNodeTypes
   props: VNodeProps | null
   key: string | number | null
@@ -221,7 +228,7 @@ export function createBlock(
 }
 
 export function isVNode(value: any): value is VNode {
-  return value ? value._isVNode === true : false
+  return value ? value.__v_isVNode === true : false
 }
 
 export function isSameVNodeType(n1: VNode, n2: VNode): boolean {
@@ -344,7 +351,8 @@ function _createVNode(
   }
 
   const vnode: VNode = {
-    _isVNode: true,
+    __v_isVNode: true,
+    __v_skip: true,
     type,
     props,
     key: props && normalizeKey(props),
@@ -403,7 +411,8 @@ export function cloneVNode<T, U>(
   // This is intentionally NOT using spread or extend to avoid the runtime
   // key enumeration cost.
   return {
-    _isVNode: true,
+    __v_isVNode: true,
+    __v_skip: true,
     type: vnode.type,
     props,
     key: props && normalizeKey(props),