Compatibility with GitHub's official Pull Request merge queue

[tetsuharuohzeki]

GitHub begin to experiment Merge Queue feature as limited beta.

• https://github.blog/changelog/2021-10-27-pull-request-merge-queue-limited-beta/
• https://docs.github.com/en/github/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/adding-a-pull-request-to-the-merge-queue

I think it would provide the primary feature of this bot. So we will get a future that we can use merge queue without setting up a kind of this bot if it's truly useful.

Then, how do this bot deal with its feature? We need to consider about it.

I think it might be better to add "work with github's merge queue" mode, but it's still in limited beta and I don't check whether it is exposed on GitHub API. It would be better to wait a stable release.