Deprecate onScrollKeyDown, refactor Flatlist selection logic

[Saadnajmi]

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

Closes #501

With #1258 , #1267, and (for posterity) #1270 , we "revived" the Flatlist prop enableSelectionOnKeyPress. This would add some custom keyboard handling to Flatlist allowing you to move through the list with arrow keys and keyboard shortcuts, and "select" a row. The prop was meant to be opt-in, meaning if it was off, then Flatlist would do no keyboard handling and let all keyboard events bubble as usual. We did this by setting a prop on the Flatlists' inner RCTScrollView: onScrollKeyDown. When that is set, RCTScrollView will handle some keyboard events (ArrowUp, ArrowDown, PageUp, PageDown, Home, End) instead of letting them bubble up through the native hierarchy. One thing to note: onScrollKeyDown has been marked for deprecation for a while now, since it doesn't conform to the agreed-upon RN keyboarding API that the desktop platforms set.

As it turns out, enableSelectionOnKeyPress wasn't truly opt-in. As it was written, RCTScrollView would always block those keyboard events natively, regardless of whether the prop was set or not. This caused some bugs where come keyboard events stopped getting handled for our partner teams.

How do we fix this? I chose to scrap the custom implementation of onScrollKeyDown altogether, and just implement that standard onKeyDown / validKeysDown API that React Native macOS has on RCTView. This is aided by the refactor done in #1338 , which made it much easier to share the keyboarding API :) With this new API, we can conditionally pass validKeysDown based on whether enableSelectionOnKeyPress is set, and make the prop truly opt in.

Changelog

[macOS] [Fixed] - Deprecate onScrollKeyDown

Test Plan

I have an (ugly..) example a View inside each FlatList row that captures ArrowUp and ArrowDown. With and without the enableSelectionOnKeyPress prop set, the view can still capture the arrow keys. I don't really want to commit this example, because it's ugly...

Screen.Recording.2022-08-24.at.1.14.09.AM.mov

[Saadnajmi]

One more request from the feature team: they would like to not have the first row selected by default. I'll add an "initialSelectedIndex' prop like the existing 'initialScrollIndex' and let it be set to undefined to accomodate.