They're just considered a
New App according to this Apple page: https://developer.apple.com/app-store/imessage-app-submissions/
It looks like they're just considered Apps for the purpose of submission on iTC. It states, in part:
Sticker pack app. This is a basic sticker pack made up of images or animated images, which you don’t need to use code to build. You can list it in the Stickers category and in relevant Stickers subcategories on the App Store for iMessage.
iMessage app. Your app can include stickers, text, video and audio. You can also use code to add functionality such as Apple Pay and in-app purchase. Choose to list your app in the Stickers category or another relevant category on the App Store for iMessage.
I also found this info on their FAQ page:
How do I create an iMessage app or sticker pack in iTunes Connect?
From My Apps, click on the plus sign and choose New App. Then, select iOS app and complete the required metadata. You can find more detailed instructions in the Creating an iTunes Connect Record for an App section of the iTunes Connect Developer Guide. If you are unfamiliar with the Messages framework, but want to learn how to get started, you can read about it here.