zustand/context: drop initialStore and add createStore

[dai-shi]

#402 (comment)

I think initialStore can easily be misused, and requires memoization on caller.

[dai-shi]

@Munawwar Are you around?

[Munawwar]

@dai-shi I was away for a while. I have to recollect the next.js issues again. I will try this new solution on the next.js example

[Munawwar]

Ok I managed to get next.js working with new interface. https://codesandbox.io/s/nextjs-with-zustand-354-forked-bryjr?file=/lib/store.js
So that's great. 👍

Though once again I thought createStore would create store every time. So it is more like createStoreOnce or createInitialStore.

[dai-shi]

Thanks for confirming.

About naming, I thought about adding Initial, but thought maybe obvious. Let's wait and see if others report confusion.