`get_store_value` doesn't account for RxJS observables

[Conduitry]

Somewhat adjacent to sveltejs/sapper#781 but I think there's other stuff going on there too.

The support we added for RxJS observables (i.e., allowing for the .subscribe() method to also return an object with an .unsubscribe() method) does not look like it extended to get_store_value.

[bvl12]

Here is a minimal repl that demonstrates the bug I was discussing in sveltejs/sapper#781. https://svelte.dev/repl/8bf28363bde34ecb8fa4e9b91a9e397a?version=3.6.3

(I had misdiagnosed the problem earlier and it didn't deal with sapper, so I have closed it)