Store resubscriptions not being generated

[Conduitry]

<script>
	import { writable } from 'svelte/store';
	let foo = writable(0);
	foo = writable(42);
</script>

{$foo}

This doesn't generate a resubscription upon the reassignment to foo. I believe this is a bug. I'm not sure what distinguishes the situation here from the one in #2014 (implemented in #2099), which has a passing unit test.

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Adjacent issue: #2181. In addition to emitting appropriate resubscription code, perhaps in cases where foo is initialized to something we can tell is not a store, we could skip the (re)subscription call. I haven't thought this part all the way through.

[Conduitry]

My guess is that #2099 only covered reassignments that happen in the template, not in the script tag.