Set endpoint methods only on `octokit.rest.*`. Export new `restEndpointMethodsLegacy` to ease transition

part of https://github.com/octokit/octokit.js/pull/15

This is a breaking change.

Removes the legacy of rest endpoint methods on `octokit.*`. Add a `restEndpointMethodsLegacy` export for `@octokit/rest`, `@octokit/actions` and others to make the change easier to absorb.