Minor: mark reducer arguments as Readonly

ts/state/ducks/items.ts

@@ -109,8 +109,8 @@ function getEmptyState(): ItemsStateType {
 }
 
 export function reducer(
-  state: ItemsStateType = getEmptyState(),
-  action: ItemsActionType
+  state: Readonly<ItemsStateType> = getEmptyState(),
+  action: Readonly<ItemsActionType>
 ): ItemsStateType {
   if (action.type === 'items/PUT_EXTERNAL') {
     const { payload } = action;