[ios] Migrate all remaining native code

[ios14] Get the app store client compiling & running on iOS 14 (#8938)

The main compiler-breaking change was the addition of a new notification option ("ephemeral notifications" for App Clips) that

[ios14] Get the app store client compiling & running on iOS 14 (#8938)

The main compiler-breaking change was the addition of a new notification option ("ephemeral notifications" for App Clips) that we needed to declare in a switch-case statement. Also left some comments for where we want to use some new enum values that replace alerts.

# Test Plan
The app compiles for iOS 14 with Xcode 12 beta. I needed to run it twice -- the first time it got stuck on the launch screen with the spinner -- and was able to log into my account and load published NCL. Images don't show up.

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[expo-notifications] Handling notifications (#6796)

# Why

Next `expo-notifications` feature.

# How

- `NotificationsHandlerModule` registers at singleton for new notifications/messages
- fo

[expo-notifications] Handling notifications (#6796)

# Why

Next `expo-notifications` feature.

# How

- `NotificationsHandlerModule` registers at singleton for new notifications/messages
- for each message it _starts up_ a task which emits an event to JS
- in response to the JS event, delegate responds with the appropriate behavior (eg. `shouldShowAlert: true`)
- the behavior is pushed to native side using `NotificationsHandler.handleNotificationAsync` call
- which directs it to the appropriate task
- task handles the behavior (on iOS calls `completionHandler`, on Android it will show the notification once implemented) and finishes
- if for whatever reason delegate didn't respond in 3 seconds, `onTimeout` is called on task, which emits another event to JS (for debugging purposes) and the task finishes


# Test Plan

Tested manually by sending notifications and logging messages that the scheme works both when the delegate responds and when it does not.

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