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/index.ts

@@ -11,13 +11,14 @@ export {
 } from './ref'
 export {
   reactive,
-  isReactive,
-  shallowReactive,
   readonly,
+  isReactive,
   isReadonly,
+  isProxy,
+  shallowReactive,
   shallowReadonly,
-  toRaw,
-  markNonReactive
+  markRaw,
+  toRaw
 } from './reactive'
 export {
   computed,