feat(nuxt): deprecate old `$nuxt` context

feat(nuxt): deprecate old `$nuxt` context

BREAKING CHANGE: Starting on this version, `@pinia/nuxt` only works with
Nuxt 2 + Bridge and Nuxt 3, it no longer works with Nuxt 2 only. This is
necessary to have one single plugin that works well with the different
versions of Nuxt. If you aren't using bridge with Nuxt 2, check out the
[migration guide](https://v3.nuxtjs.org/bridge/overview) or pin your
`@pinia/nuxt` dependency in your:

```diff
-    "@pinia/nuxt": "^0.2.1",
+    "@pinia/nuxt": "0.2.1",
```

The `$nuxt` context usage should be replaced with globals like
`$fetch()` and `useNuxtApp()`. You can find more information about this
in Nuxt documentation:

- https://v3.nuxtjs.org/bridge/bridge-composition-api/
- https://v3.nuxtjs.org/bridge/overview