BREAKING CHANGE: template auto ref unwrapping are now applied shallowly,
i.e. only at the root level.

  This change aims to ensure that non-ref values referenced in templates
  retain the same identity with the value declared inside `setup()`
  regardless of whether it's wrapped with `reactive` or `readonly`.

  The breaking case is that given the following:

  ```js
  setup() {
    return {
      one: ref(1),
      two: {
        three: ref(3)
      },
      four: 4
    }
  }
  ```

  After this change, `{{ one }}` in the template will still render `1`,
  but `{{ two.three }}` will no longer be auto unwrapped as `3`.

  A common case where this could happen is returning an object of refs
  from a composition function. The recommendation is to either expose
  the refs directly, or call the newly exposed `proxyRefs` on the object
  so that the object's refs are unwrapped like root level refs. In fact,
  `proxyRefs` is also the method used on the `setup()` return object.

  This also makes non-ref properties in the returned objects non-reactive,
  so mutating `four` from the template will no longer trigger updates.