feat(reactivity): public contract for ref, reactive (fix vuejs/rfcs#129)

[jods4]

This PR adds two new public functions: markRef and markReactive.
They are meant for advanced users or library authors and are only exported by @vue/reactivity, not vue itself.

The use-case here is to allow libraries to provide alternative, specialized implementations of the ref (observable value) and reactive (observable object) concepts.

Example use cases

One could want to create a ref that debounces its notifications:

function debouncedRef<T>(delay: number, value?: T) {
  let timeout = 0
  return markRef({
    get value() {
      track(this, TrackOpTypes.GET, 'value')
      return value
    },
    set value(v: T) {
      value = v
      clearTimeout(timeout)
      timeout = setTimeout(() => trigger(this, TriggerOpTypes.SET, 'value'), delay)
    }
  })
}

On the reactive side, one could want to create:

• an implementation based on getter/setters rather than proxies (e.g. to work with classes using private fields);
• an optimized reactive array that doesn't watch changes individually but as a whole;
• "fake" reactive objects that do limited functionality, e.g. hideRefs({ a: ref(4) }) that only automatically unwrap refs.

Alternatives

It is possible to mark alternative implementations with markNonReactive but it doesn't integrate as well with Vue and some scenarios are broken.

For example, you can't make a Ref<T> in Typescript because the interface uses a private symbol to prevent that. This can only be worked around with <any> casts.

On the reactive side, some operations use toRaw (notably the template ref feature, e.g. <Component ref='x'>) and this is not exposed publicly so there is no way to tap into it.

RFC

Closes vuejs/rfcs#129

[jods4]

One test is failing in CI but it's not related to my changes, it's the markdown spec in examples.

[yyx990803]

Closing per discussion in vuejs/rfcs#157