Refine Keyboard API Event typings (#23272)

Summary:
- Improve type inference of the `event.easing` keyboard event property with the `KeyboardEventEasing` type.
- Exporting `KeyboardEventName` and `KeyboardEventEasing` for others to use.
- Removing unnecessary optional property flag (ie.`?`) for all properties in `KeyboardEvent`. (I personally haven't seen any of the properties not being returned for all keyboard events).

[iOS] [Changed] - Refine Keyboard API Event typings
Pull Request resolved: https://github.com/facebook/react-native/pull/23272

Differential Revision: D13941758

Pulled By: cpojer

fbshipit-source-id: 4891898c29cf6963069dbe6b4d439694761fd9e7

Libraries/Components/Keyboard/Keyboard.js

@@ -17,7 +17,7 @@ const KeyboardObserver = require('NativeModules').KeyboardObserver;
 const dismissKeyboard = require('dismissKeyboard');
 const KeyboardEventEmitter = new NativeEventEmitter(KeyboardObserver);
 
-type KeyboardEventName =
+export type KeyboardEventName =
   | 'keyboardWillShow'
   | 'keyboardDidShow'
   | 'keyboardWillHide'
@@ -25,6 +25,13 @@ type KeyboardEventName =
   | 'keyboardWillChangeFrame'
   | 'keyboardDidChangeFrame';
 
+export type KeyboardEventEasing =
+  | 'easeIn'
+  | 'easeInEaseOut'
+  | 'easeOut'
+  | 'linear'
+  | 'keyboard';
+
 type ScreenRect = $ReadOnly<{|
   screenX: number,
   screenY: number,
@@ -33,11 +40,11 @@ type ScreenRect = $ReadOnly<{|
 |}>;
 
 export type KeyboardEvent = $ReadOnly<{|
-  duration?: number,
-  easing?: string,
+  duration: number,
+  easing: KeyboardEventEasing,
   endCoordinates: ScreenRect,
-  startCoordinates?: ScreenRect,
-  isEventFromThisApp?: boolean,
+  startCoordinates: ScreenRect,
+  isEventFromThisApp: boolean,
 |}>;
 
 type KeyboardEventListener = (e: KeyboardEvent) => void;