Flow: textShadownOffest needs to be stricter

Summary: `width` and `height` are required for this style prop, for the sake of consistency (and Fabric). Callsites should set one of them to 0 instead of not specifying it.

Reviewed By: TheSavior, shergin

Differential Revision: D8371064

fbshipit-source-id: b0ffd6b6543ac5456a3708382966e7b3df241f7e

Libraries/StyleSheet/StyleSheetTypes.js

@@ -185,8 +185,8 @@ export type ____TextStyle_Internal = $ReadOnly<{|
     | 'proportional-nums',
   >,
   textShadowOffset?: $ReadOnly<{|
-    width?: number,
-    height?: number,
+    width: number,
+    height: number,
   |}>,
   textShadowRadius?: number,
   textShadowColor?: ColorValue,