feat: Add CODEOWNERS file

[koji-1009]

ref https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 3ca6bcc):

https://flutterkakgi-2023-conf-app-dev--pr98-feat-codeowner-dxm9o3x8.web.app

_{(expires Fri, 08 Sep 2023 23:07:58 GMT)}

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[koji-1009]

@FlutterKaigi/conference-app-2023 レビューをお願いします。publicリポジトリにした後、CODEOWNERのレビューをマージに必須にするための対応です。