Improve types for subscribe callback return values

[appden]

This helps better "document" the optional cleanup callback behavior as well as catch potential mistakes. The React useEffect, et al hook types similarly have these typing constraints, so this helps with consistency.

Note: I think this is nice to have, but won't be offended at all if you disagree and just close this PR. 😀

[onurkerimov]

Hey @appden , at the first glance I was unsure of merging this, but I didn't want to dismiss it. Now, after experimenting more and more with xoid, I definitely agree it's worth adding. I'm very thankful for your contributions to this project. All of them were good ones, and some of them were essential to the core. Also I'm wondering what awesome thing you've built using xoid 🙂

Soon I'm going to announce this project with a Medium post. I'd appreciate any suggestion/feedback of yours on this. Best!

[appden]

Thanks @onurkerimov, I really appreciate your kind words and your openness to contributions!

I've been (very slowly) building something in the AI image generation space, but it's far from awesome at this point. I love the simplicity and flexibility of Xoid atoms, so I've been using them in a variety of ways to achieve kinda-sorta fine-grained reactivity through my codebase, which is especially helpful for React Native apps with multiple roots (thus can't share context).