Update the project's dependency from isomorphic-fetch to cross-fetch

### Context
To prevent the `Module not found: Can't resolve 'unfetch'` error, replace [`isomorphic-fetch`](https://github.com/matthew-andrews/isomorphic-fetch) (not actively maintained) with [`cross-fetch`](https://github.com/lquixada/cross-fetch).

We'll address this issue before the Next.js app router migration begins.

### Problem or idea
Update the project dependency listed above.

### Solution or next step
- Add [`cross-fetch`](https://github.com/lquixada/cross-fetch)
- Remove [`isomorphic-fetch`](https://github.com/matthew-andrews/isomorphic-fetch)
- Adjust the import statements