[iOS] Add support for Apple Focus Groups (UIFocusGroup) for keyboard navigation

[aleksandraduczekwebsensa]

Apple introduced the Focus System on iPadOS 15 and made the UIFocusGroup API public starting with iOS/iPadOS 16.
This API allows developers to define logical groups of focusable elements.
When a hardware keyboard or trackpad is used:
Arrow keys move focus within a group.
Tab / Control + Tab moves between focus groups.
This is the same behavior seen in many system apps (e.g., Settings, Files, and Safari).

Currently, React Native does not expose any APIs to:
Register a view as a focus group container.
Control or query focus movement via Apple’s Focus System.
Listen to UIFocusUpdateContext or related native focus events.

As a result, keyboard navigation on iOS feels inconsistent compared to native UIKit or SwiftUI apps.

Proposal: Add a React Native API that integrates with UIKit’s Focus System.

References:
Apple Documentation: UIFocusGroup
WWDC 2021 Session 10058: “Focus improvements for iPadOS apps”
Apple UIKit Reference: UIFocusSystem Overview