Restore the State export in the types

Author: timdorr

packages/history/index.ts

@@ -48,6 +48,14 @@ export type Search = string;
  */
 export type Hash = string;
 
+/**
+ * An object that is used to associate some arbitrary data with a location, but
+ * that does not appear in the URL path.
+ *
+ * @see https://github.com/ReactTraining/history/tree/master/docs/api-reference.md#location.state
+ */
+export type State = object | null;
+
 /**
  * A unique string associated with a location. May be used to safely store
  * and retrieve data in some other storage API, like `localStorage`.