(feat) control flow for stores

[dummdidumm]

#493
$store --> (__svelte_store_get(store), $store)
By using the comma operand we allow TypeScript's control flow to work. Before that, it was $store --> __svelte_store_get(store) which loses control flow info every time.

We need to wait for TS 4.2 before we can merge this because we need a not-yet-published control flow fix for this solution to work.
I tested rename, go-to-declaration, find-references, hover - all looks good. But a second look once this is finalized would be good.

Props to @GrzegorzKazana from which most of this code is - thanks!

[GrzegorzKazana]

This is great - can't wait for it to be released! 🚀
Kudos for not giving up on this approach 😅

[dummdidumm]

Yes, my objection in the now closed PR of yours "this won't work because X" was actually unjustified, because my code example was wrong: It said let $bla: {...sometype} = ..., but it's let $bla = __svelte... which means TS is smart enough to defer all the type intellisense to bla. So most of your code was already right and working. And since TS 4.2 will make (get(bla), $bla).blubb possible with type narrowing (I just checked with TS nightly and can confirm it works), we can ditch the more cumbersome (get(bla), $bla.blubb)-solution.
So now we just wait 😄