feat(sequelize): add belongsToMany definition

sequelize/sequelize.d.ts

@@ -909,6 +909,14 @@ declare module "sequelize"
              */
             belongsTo<TInstance, TPojo>(target: Model<TInstance, TPojo>, options?: AssociationOptions): void;
 
+            /**
+             * Creates an association to connect sources with multiple targets. Furthermore the targets can also have connections to multiple sources.
+             *
+             * @param target
+             * @param options
+             */
+            belongsToMany<TInstance, TPojo>(target: Model<TInstance, TPojo>, options?: AssociationOptions): void;
+
             /**
              * Create an association that is either 1:m or n:m.
              *