ios [nfc]: Cut notifications library out of handling "initial notification"

[chrisbobbe]

I'm thinking that for iOS notifications in zulip-flutter (zulip/zulip-flutter#122, etc.), it'll help to demonstrate a working implementation that doesn't depend on @react-native-community/push-notification-ios, since that library's code can be unnecessarily opaque, and also buggy.

#5676 took a first step toward that, and this takes another step. (The former was about tapping a notification when the app is already open; this PR is in the code for tapping a notification when the app is closed.)

[chrisbobbe]

Tested on my iPhone 13 Pro running iOS 16.