Support RxJS Observables in reassigned stores

[Conduitry]

Fixes #3304.

This also refactors stuff a little bit. The subscribe helper is now called component_subscribe (for lack of a better name), and there's a new subscribe helper that just does subscription and returns the unsubscription function, whether this is a Svelte store or an RxJS observable. This new helper is now used in component_subscribe, get_store_value, and in the store reassignment/resubscription handling code.

[Conduitry]

I think with this PR the only place left where we're blindly calling .subscribe() and assuming it'll directly return an unsubscription function is in derived. But that's so tied to our other store implementations now that I'm guessing it doesn't make sense to try to make it work with Observables.

[Rich-Harris]

agree with that assessment, RxJS people have a gazillion ways to do the same thing