add webp support for typescript (facebook#5978)

* add webp support for typescript

* moved webp declaration below png

Author: dugagjin

Diff for: lib/react-app.d.ts

@@ -34,6 +34,11 @@ declare module '*.png' {
   export default src;
 }
 
+declare module '*.webp' {
+    const src: string;
+    export default src;
+}
+
 declare module '*.svg' {
   import * as React from 'react';