isomorphic-unfetch: Consider making node-fetch a peerDependency

I have the case where I'd like to have better control of the version of `node-fetch` used (v3.x beta). If it were a peer dependency I'd be able to do this.

Start by inspecting the package metadata and dependency setup to see how node-fetch is currently declared and selected. Review the existing test entry points, if present, then verify that consumers can control the node-fetch version through the peer-dependency arrangement without breaking supported usage.

Written by the indexing model from the issue text.