fix: useAsync bug with Promises

[nvitaterna]

🐛 The bug
UseAsync checks for x instanceof Promise which isn't completely reliable. Should probably check for .then instead to cover other Promise libraries. This makes Nuxt not fetch the data.

🛠️ To reproduce
For example, using the Nuxt Content Module:

// doesn't work
const article = useAsync(() =>
  $content('articles', params.value.slug).fetch<ArticleResult>()
);

//works
const article = useAsync(() =>
  Promise.resolve(
    $content('articles', params.value.slug).fetch<ArticleResult>()
  )
);

This is because, for whatever reason, the .fetch() function from the content module does not return an instanceof Promise.

🌈 Expected behaviour
Expect it to fetch the data on the server.