chore: Add focusable to macOS view config (#2638)

## Summary:

Fix a couple of static view config validation warnings by adding
`focusable` to the macOS view config.

## Test Plan:

Less warnings in RNTester when I launch it.

Author: Saadnajmi

packages/react-native/Libraries/NativeComponent/BaseViewConfig.macos.js

@@ -52,6 +52,7 @@ const validAttributesForNonEventProps = {
   cursor: true,
   draggedTypes: true,
   enableFocusRing: true,
+  focusable: true,
   tooltip: true,
   keyDownEvents: true,
   keyUpEvents: true,