Add definition for React DOM SyntheticPointerEvent

This is based on facebook/react#12507 which is
seemingly missing a couple of fields from the official spec.
Consequently this will need an update once React supports all fields.

lib/react-dom.js

@@ -224,6 +224,19 @@ declare class SyntheticWheelEvent<
  deltaZ: number;
 }
 
+declare class SyntheticPointerEvent<
+ +T: EventTarget = EventTarget,
+> extends SyntheticMouseEvent<T> {
+ pointerId: number,
+ width: number,
+ height: number,
+ pressure: number,
+ tiltX: number,
+ tiltY: number,
+ pointerType: string,
+ isPrimary: boolean,
+}
+
 declare class SyntheticTouchEvent<
  +T: EventTarget = EventTarget,
 > extends SyntheticUIEvent<T> {