export ActionHandler (#1201)

- while testing we have methods using this method signature and it would be nice if we did not have define this.

We were running into the issue as reported in microsoft/TypeScript#7960

So workaround was to do something like
```
const loginAction = userActions.login as ModuleAction<UserState>;
```

Author: sks

types/index.d.ts

@@ -86,8 +86,8 @@ export interface StoreOptions<S> {
   strict?: boolean;
 }
 
-type ActionHandler<S, R> = (injectee: ActionContext<S, R>, payload: any) => any;
-interface ActionObject<S, R> {
+export type ActionHandler<S, R> = (injectee: ActionContext<S, R>, payload: any) => any;
+export interface ActionObject<S, R> {
   root?: boolean;
   handler: ActionHandler<S, R>;
 }