Improve VoiceOver accessibility support for Flatlist

[chiuam]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

This change improves the VoiceOver accessibility experience in Flatlist by adding support for announcing the row index.
Note: When navigating into the "table" and selecting the first row item, VO announces "No selection" before its label. I couldn't find a good workaround for this unwanted behavior.(Created #1348 for tracking)

Changelog

[macOS] [Fixed] - Improve VoiceOver accessibility support for Flatlist

Test Plan

Before:
VO announces the ScrollView element as scroll area
VO+R is no-op
keyboard focus / VO+fn+3 announces the item label

Before.mov

After:
VO announces the ScrollView element as table
keyboard focus / VO+R announces the item label
VO+fn+3 announces the item label and its index

After.mov

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: 6fcfe2e
Branch: main

[harrieshin]

does the flatlist automatically scroll when you change the VO focus on one of the subview?

[chiuam]

does the flatlist automatically scroll when you change the VO focus on one of the subview?

Nope, which is bad because it deviates from the native behavior :/
Edit: Created #1347 for tracking