refactor(reactivity): adjust APIs

BREAKING CHANGE: Reactivity APIs adjustments:

- `readonly` is now non-tracking if called on plain objects.
  `lock` and `unlock` have been removed. A `readonly` proxy can no
  longer be directly mutated. However, it can still wrap an already
  reactive object and track changes to the source reactive object.

- `isReactive` now only returns true for proxies created by `reactive`,
   or a `readonly` proxy that wraps a `reactive` proxy.

- A new utility `isProxy` is introduced, which returns true for both
  reactive or readonly proxies.

- `markNonReactive` has been renamed to `markRaw`.

packages/reactivity/src/ref.ts

@@ -1,7 +1,7 @@
 import { track, trigger } from './effect'
 import { TrackOpTypes, TriggerOpTypes } from './operations'
 import { isObject } from '@vue/shared'
-import { reactive, isReactive } from './reactive'
+import { reactive, isProxy } from './reactive'
 import { ComputedRef } from './computed'
 import { CollectionTypes } from './collectionHandlers'
 
@@ -98,7 +98,7 @@ export function customRef<T>(factory: CustomRefFactory<T>): Ref<T> {
 export function toRefs<T extends object>(
   object: T
 ): { [K in keyof T]: Ref<T[K]> } {
-  if (__DEV__ && !isReactive(object)) {
+  if (__DEV__ && !isProxy(object)) {
     console.warn(`toRefs() expects a reactive object but received a plain one.`)
   }
   const ret: any = {}