`KVNamespaceListResult` more specific types

[DaniFoldi]

The two possibilities of

• the listing being complete, list_complete: true and cursor is an empty string, or undefined (not tested)
• there are more elements to be returned, list_complete: false and cursor is a string

With a union type of these two, TS compiler would know which case we are in, if we check list_complete, so cursor wouldn't have to be rechecked.

Docs: https://developers.cloudflare.com/workers/runtime-apis/kv/#pagination

Interface definition: https://github.com/cloudflare/workers-types/blob/38b7e0fba83f01654a00b0d805cd01211a419f3d/overrides/kv.d.ts#L17-L21