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> {