Define a more generic callback type that returns unknown instead of void

[nicu-chiciuc]

The package has implicit type for get, all, etc.

The type of get is inferred as:

(method) RedisStore.get(sid: string, cb?: (_err?: unknown, _data?: any) => void): Promise<void>

Whereas based on the implementation the get can be awaited and a value can be returned.

A better approach would've been to use generics, but for now a simple change to unknown from void is more than acceptable.

[github-actions]

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[wavded]

The use of promises here is really just to await the Redis implementations we use (so only intended for internal use to keep the code cleaner). The actual public API follows the spec of using Node-style callbacks defined within the express-session store spec. Promises don't return anything meaningful to the end user and are not intended to be used. I suppose we could run async functions within the functions such that TS type reporting isn't confusing.