Multi-search response does not include `length`

[GauthierPLM]

The return type of multiSearch.perform() specifies that a length field should be included, but this field is not returned by TypeSense and not listed in the documentation.

Type declaration: https://github.com/typesense/typesense-js/blob/master/src/Typesense/MultiSearch.ts#L57
Documentation: https://typesense.org/docs/29.0/api/federated-multi-search.html#federated-search

The field should be either added to match the expected behavior or removed from the type declaration.