Draft State for Pull Requests

Would be nice to have something similar to https://github.blog/2019-02-14-introducing-draft-pull-requests/.

A useful snippet: “A draft pull request is styled differently to clearly indicate that it’s in a draft state. Merging is blocked in draft pull requests. Change the status to “Ready for review” near the bottom of your pull request to remove the draft state and allow merging according to your project’s settings. Also, if you have a CODEOWNERS file in your repository, a draft pull request will suppress notifications to those reviewers until it is marked as ready for review.”