Use Objective-C nullability to provide better Swift types in RCTAppDelegate

[okwasniewski]

Summary:

This PR adds nullable type annotations + nullability audit regions (NS_ASSUME_NONNULL_BEGIN/NS_ASSUME_NONNULL_END) to provide better Swift types.

Before:

class AppDelegate: RCTAppDelegate {
  override func sourceURL(for bridge: RCTBridge!) -> URL! {
    self.bundleURL()
  }
  
  override func bundleURL() -> URL! {
    RCTBundleURLProvider.sharedSettings().jsBundleURL(forBundleRoot: "js/RNTesterApp.ios")
  }
  
  override func customize(_ rootView: RCTRootView!) {
    rootView.backgroundColor = .red
  }
}

After:

class AppDelegate: RCTAppDelegate {
  override func sourceURL(for bridge: RCTBridge) -> URL {
    self.bundleURL()
  }
  
  override func bundleURL() -> URL {
    RCTBundleURLProvider.sharedSettings().jsBundleURL(forBundleRoot: "js/RNTesterApp.ios")
  }
  
  override func customize(_ rootView: RCTRootView) {
    rootView.backgroundColor = .red
  }
}

Changelog:

[IOS] [ADDED] - Provide better Swift types for RCTAppDelegate and RCTBridgeDelegate

Test Plan:

In Xcode click the top left square button and look up the generated interface:

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@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	16,769,588	-764
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	20,165,562	+3,740
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: 8fd2f12
Branch: main

[github-actions]

This pull request was successfully merged by @okwasniewski in a7c5c28.

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