Adds node-fetch as an optional dependency

[eoingroat]

This allows it to import node-fetch in a pnp / yarn berry environment

Otherwise this fails with an error like the following:

fetch-mock tried to access node-fetch (a peer dependency) but it isn't provided by its ancestors; this makes the require call ambiguous and unsound.

As an optionalDependency, and not a peerDependency, it does not have to be satisfied.

[ljharb]

That’s not how optionalDependencies work; those are for things that might fail on install, and should never be set to *.

You may want a peer dep, using peerDependenciesMeta to mark it optional, but tbh this is a design flaw in pnp: presence on disk, not presence in package.json, is how require works.

All that said, it makes zero sense to mock fetch if it’s not in the dep tree, so what i think you actually want here is to list it as both a dep and a (required) peer dep.