fix(reactive): use isExtensible instead of isFrozen

[wujieZ]

use isExtensible instead of isFrozen

[yyx990803]

A non-extensible object may still be mutable, and can be observed.

Also Vue 2 uses the same Object.isFrozen check and Vue 3 should keep it consistent.

[wujieZ]

thanks for commented! ! but....

1. when I try to reactive a non-extensible object

const { reactive } = Vue
const obj = reactive(Object.preventExtensions({a: 1}))

it will cause an error, beacuse a non-extensible object may not be a Frozen obejct, so we try to define a property(__v_reactive) for a non-extensible object in source code.

- Uncaught TypeError: Cannot define property __v_reactive, object is not extensible
    at Function.defineProperty (<anonymous>)
    at def (vue.global.js:327)
    at createReactiveObject (vue.global.js:970)
    at reactive (vue.global.js:927)
    at setup (test.html:13)
    at callWithErrorHandling (vue.global.js:1277)
    at setupStatefulComponent (vue.global.js:6814)
    at setupComponent (vue.global.js:6775)
    at mountComponent (vue.global.js:5107)
    at processComponent (vue.global.js:5083)

2. In Vue2, we also use Object.isExtensible to check。

export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&     //   use Object.isExtensible to check
    !value._isVue
  ) {
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}