feat: add support for splitting focus on nested views of iOS

[AKB48]

Summary

This PR #27855 allows nested views like illustration shows below to obtain three focus when accessibilitySplitFocus is set on the parent view.
This PR aims to make accessibility focus acts as consistent with that on Android for these nested views.
When touch point is inside of the inner element, the corresponding inner element would be activated other than the outer one.
If a merged focus is needed, just don't set it or set accessibilitySplitFocus to false.

 ---------------------
|   Outer Element  <--|---focus 1
|                     |
|  --------------- 
| | Inner Element |<--|---focus 2
|  ---------------    |
|  ---------------    |
| | Inner Element |<--|---focus 3
|  ---------------    |
|                     |
 ---------------------

Changelog

[iOS][Added] - Added accessibilitySplitFocus prop to support focus capturing of subviews of nested views.

Test Plan

[1] Run RNTester
[2] Go to the "AccessibilityIOS" page
[3] Toggle VoiceOver in settings.
[4] When accessibilitySplitFocus is not set:
[4-1] Touch on the "First Inner Element", only get the merged focus(outer one)
[4-2] Touch on the "Second Inner Element", get the merged focus as well
[5] When accessibilitySplitFocus is set:
[5-1] Touch on the "First Inner Element", get focus of the first inner element
[5-2] Touch on the "Second Inner Element", get focus of the second one
[5-3] Touch on the space out of two inner elements but inside the outer element, get the merged focus

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	6,762,741	-382
android	hermes	armeabi-v7a	6,425,668	-246
android	hermes	x86	7,149,225	-204
android	hermes	x86_64	7,039,125	-236
android	jsc	arm64-v8a	8,936,107	-398
android	jsc	armeabi-v7a	8,591,411	-263
android	jsc	x86	8,765,713	-221
android	jsc	x86_64	9,341,247	-268

Base commit: 56689e9

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	10,786,272	9,956,832

Base commit: 56689e9