Add a getItem function.

source/item/item.ts

@@ -119,6 +119,23 @@ export async function createItem(
   return item;
 }
 
+/**
+ * Get the item for a given key and {@link ItemKeyPrefix}. Note that this
+ * function assumes that the item definitely exists.
+ */
+export async function getItem(
+  key: string,
+  itemKeyPrefix: ItemKeyPrefix,
+): Promise<Value<Item>> {
+  return createValue<Item>({
+    deserialize: deserializeItem,
+    serialize: serializeItem,
+    storage: storageForPrefix(itemKeyPrefix),
+    key,
+    value: undefined!,
+  });
+}
+
 /**
  * Get all keys from storage that start with the {@link ItemKeyPrefix}.
  *