Add Sticker Pack Extension

[uhooi]

Distribute the sticker pack.

References:

• https://developer.apple.com/imessage/
• https://developer.apple.com/jp/imessage/
• https://developer.apple.com/jp/app-store/review/guidelines/
  4.4.3 ステッカー
• https://developer.apple.com/documentation/messages
• https://stackoverflow.com/questions/39506728/ios-10-add-sticker-extension-to-an-existing-application
• http://instantsentimental.com/2016/10/26/imessage-app-sticker-extension-%E3%82%92%E4%BD%9C%E3%82%8D%E3%81%86%E3%81%A8%E3%81%97%E3%81%A6%E3%83%8F%E3%83%9E%E3%81%A3%E3%81%9F%E3%83%A1%E3%83%A2/
• http://ichigoro.hatenablog.com/entry/2016/09/22/113610
• https://github.com/yonaskolb/XcodeGen/issues/727
• https://github.com/yonaskolb/XcodeGen/blob/474d8f69076876fe4eb1ff5e7534af74e9c5be9f/Tests/Fixtures/TestProject/project.yml
• https://github.com/scelis/XcodeGen-TestStickers/blob/2b146cfc9e7ae37e9d02819a9eb0630d4f17e7d0/project.yml

[github-actions[bot]]

👋 Thanks for reporting! Please PR :)