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/reactivity/src/ref.ts

@@ -5,18 +5,11 @@ import { reactive, isProxy, toRaw } from './reactive'
 import { ComputedRef } from './computed'
 import { CollectionTypes } from './collectionHandlers'
 
-const isRefSymbol = Symbol()
-
 export interface Ref<T = any> {
-  // This field is necessary to allow TS to differentiate a Ref from a plain
-  // object that happens to have a "value" field.
-  // However, checking a symbol on an arbitrary object is much slower than
-  // checking a plain property, so we use a _isRef plain property for isRef()
-  // check in the actual implementation.
-  // The reason for not just declaring _isRef in the interface is because we
-  // don't want this internal field to leak into userland autocompletion -
-  // a private symbol, on the other hand, achieves just that.
-  [isRefSymbol]: true
+  /**
+   * @internal
+   */
+  __v_isRef: true
   value: T
 }
 
@@ -27,7 +20,7 @@ const convert = <T extends unknown>(val: T): T =>
 
 export function isRef<T>(r: Ref<T> | unknown): r is Ref<T>
 export function isRef(r: any): r is Ref {
-  return r ? r._isRef === true : false
+  return r ? r.__v_isRef === true : false
 }
 
 export function ref<T extends object>(
@@ -51,7 +44,7 @@ function createRef(rawValue: unknown, shallow = false) {
   }
   let value = shallow ? rawValue : convert(rawValue)
   const r = {
-    _isRef: true,
+    __v_isRef: true,
     get value() {
       track(r, TrackOpTypes.GET, 'value')
       return value
@@ -99,7 +92,7 @@ export function customRef<T>(factory: CustomRefFactory<T>): Ref<T> {
     () => trigger(r, TriggerOpTypes.SET, 'value')
   )
   const r = {
-    _isRef: true,
+    __v_isRef: true,
     get value() {
       return get()
     },
@@ -126,7 +119,7 @@ export function toRef<T extends object, K extends keyof T>(
   key: K
 ): Ref<T[K]> {
   return {
-    _isRef: true,
+    __v_isRef: true,
     get value(): any {
       return object[key]
     },