fix: store access on component destroy

[paoloricciuti]

Closes #14950

While working on this initially with @oscard0m i actually found out of a separate bug, a memory leak: basically if you did something like this

<script>
    let { store } = $props();

    $effect(()=>{
        return ()=>{
            $store;
        }
    });
</script>

the store was accessed for the first time after the teardown of the stores executed. So a new subscription was created for the store that generally would've been cleaned up by the teardown but since it already ran it was stuck in memory.

This was even worse for stuff that was accessed on component destroy asynchronously.

This solution does two things:

1. the setup_stores function doesn't register the teardown immediately but returns a function that does that. This function is called after pop so that register even after every user effect.
2. now that we know that the teardown will be executed last, on teardown uses the stores variable as a way of communication...it register a symbol on it so that inside get_store we can check if the component was already unmounted and, in that case, it prevents the creation of a subscription in the store while at the same time accessing the store with get from svelte/store, less efficient but more consistent (the bug we are closing was about the consistency of the read) and not leaky. Furthermore this will only really happen when someone is accessing stuff after the component unmounts so should not be a problem.

I didn't found a way to test the memory leak but if you have ideas it could be good to add.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14968-svelte.vercel.app/

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pnpm add https://pkg.pr.new/svelte@14968

[dummdidumm]

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