Add `useReduce$`

[Bloomca]

Description

From my testing with lucy-state, I think useReduce$ is probably the most useful helper, so I think we need to introduce it as well.

It can be practically copied from https://github.com/Bloomca/react-lucy-state/blob/main/src/utils/reduce.ts, with small tests adjustment.

[Bloomca]

Okay, so implementing is pretty simple, but thinking about it, maybe it is not necessary.

These two things are equivalent:

Using combine and select:

  const nameState = createState("");
  const lastNameState = createState("");
  const fullNameState = select(
    combine(nameState, lastNameState),
    ([firstName, lastName]) => `${firstName} ${lastName}`
  ); 

And this is using reduce:

  const nameState = createState("");
  const lastNameState = createState("");
  const fullNameState = reduce(
    [nameState, lastNameState],
    ([firstName, lastName]) => `${firstName} ${lastName}`
  ); 

Honestly, the advantage is very small, so I think I'll just close this issue. I think the state system needs a rework to allow at least Rxjs to be compatible (similar to how