Move to @octokit org

[gr2m]

we decided to make this plugin official™️ @timrogers @oscard0m

• transfer repository to @octokit
• rename repository to octokit/plugin-create-pull-request
• rename package name to @octokit/plugin-create-pull-request and release new version v4
• create new repository gr2m/octokit-plugin-create-pull-request-legacy. That way https://github.com/gr2m/octokit-plugin-create-pull-request will continue to redirect to the new https://github.com/octokit/plugin-create-pull-request
• Install @octokit/plugin-create-pull-request as the sole dependency
• Add deprecation message when used
• deprecate octokit-plugin-create-pull-request-legacy package on npm
• Release a new version octokit-plugin-create-pull-request-legacy@3.14.0