Notify new character

[uhooi]

References:

• https://firebase.google.com/docs/cloud-messaging/ios/client?authuser=1
• https://firebase.google.com/docs/cloud-messaging/ios/certs?hl=ja
• https://firebase.google.com/docs/cloud-messaging/ios/receive?authuser=1#swift:-ios-10
• https://developer.apple.com/forums/thread/661953
• https://qiita.com/rockname/items/4a092e39e571ddd19c5b

[github-actions[bot]]

👋 Thanks for reporting! Please PR :)