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/componentSlots.ts

@@ -4,7 +4,7 @@ import {
   VNodeNormalizedChildren,
   normalizeVNode,
   VNodeChild,
-  InternalObjectSymbol
+  InternalObjectKey
 } from './vnode'
 import {
   isArray,
@@ -111,7 +111,7 @@ export const initSlots = (
       normalizeVNodeSlots(instance, children)
     }
   }
-  def(instance.slots, InternalObjectSymbol, true)
+  def(instance.slots, InternalObjectKey, 1)
 }
 
 export const updateSlots = (