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/runtime-core/src/index.ts

@@ -2,20 +2,24 @@
 
 export const version = __VERSION__
 export {
+  // core
+  reactive,
   ref,
+  readonly,
+  // utilities
   unref,
-  shallowRef,
   isRef,
   toRef,
   toRefs,
-  customRef,
-  reactive,
+  isProxy,
   isReactive,
-  readonly,
   isReadonly,
+  // advanced
+  customRef,
+  shallowRef,
   shallowReactive,
   shallowReadonly,
-  markNonReactive,
+  markRaw,
   toRaw
 } from '@vue/reactivity'
 export { computed } from './apiComputed'