Refactor nuxtMock object into a separate file

As discussed in [PR #697](https://github.com/druxt/druxt.js/pull/697) and [comment](https://github.com/druxt/druxt.js/pull/697#discussion_r1765907542), the `nuxtMock` object may need to be refactored into a separate file for better reusability and maintainability. This issue is opened at the request of @Decipher.