Add @octokit/core as peerDependency

This is a follow up issue to https://github.com/octokit/plugin-paginate-rest.js/issues/124#issuecomment-681019208

We need to add `@octokit/core` as a peerDependency to address a problem with yarn 2. It is something I planned to do anyway, but wanted to figure out automated testing to assure compatibility of the plugins with the latest `@octokit/core`.

I only want to define compatibility with the lowest version, in this case v3. Compare pull request https://github.com/octokit/plugin-paginate-rest.js/pull/130/files.

I would appreciate help, this is a great issue for a first-time contributor

Compare the requested dependency and compatibility approach with pull request 130, then inspect this plugin's package configuration and existing tests. Add @octokit/core with v3 as the lowest supported peer version and verify compatibility with the latest core version through the repository's test setup.

Written by the indexing model from the issue text.