Reintroduce getMany optional method (#384)

* Revert "Revert "Add missing `getMany` optional method on Store interface (#362)" (#365)"

This reverts commit da05c2f.

* null | undefined

* no null type

packages/keyv/src/index.d.ts

@@ -92,12 +92,17 @@ declare namespace Keyv {
 		value: Value; expires: number | undefined;
 	}
 
+	type StoredData<Value> = DeserializedData<Value> | string | undefined;
+
 	interface Store<Value> {
 		get(key: string): Value | Promise<Value | undefined> | undefined;
 		set(key: string, value: Value, ttl?: number): any;
 		delete(key: string): boolean | Promise<boolean>;
 		clear(): void | Promise<void>;
 		has?(key: string): boolean | Promise<boolean>;
+		getMany?(
+			keys: string[]
+		): Array<StoredData<Value>> | Promise<Array<StoredData<Value>>> | undefined;
 	}
 }