implement the full functionality of `@octokit/endpoint` in `@octokit-next/endpoint`

Including all of the README, tests, and whatever else is implemented in `@octokit/endpoint`

Two dependencies need to be upgraded to ESM

- [x] https://github.com/jonschlinkert/is-plain-object (replaced with https://github.com/sindresorhus/is-plain-obj)
- [x] https://github.com/gr2m/universal-user-agent