fix(5019): imported stores can be identified as reactive store subscr…

[skippednote]

This PR address the issue #5019.
The way stores are filtered it is expected variable to be hoistable.
https://github.com/sveltejs/svelte/blob/master/src/compiler/compile/render_dom/index.ts#L320

const reactive_store_subscriptions = reactive_stores
    .filter(store => {
        const variable = component.var_lookup.get(store.name.slice(1));
        return !variable || variable.hoistable;
    })
    .map(({ name }) => b`
        ${component.compile_options.dev && b`@validate_store(${name.slice(1)}, '${name.slice(1)}');`}
        @component_subscribe($$self, ${name.slice(1)}, $$value => $$invalidate(${renderer.context_lookup.get(name).index}, ${name} = $$value));
    `);

Since imports are by their nature hoistable, I have explicitly set that.

[Conduitry]

It looks like this marks every instance variable as hoistable, not just the imports - and this is also breaking many tests.

[skippednote]

Thank you @Conduitry for the feedback.
I've taken a different route now, I've marked imported stores as hoistable specifically. All the existing tests seem to be passing now.

[Conduitry]

I think there's another field on the variable object that ought to be able to tell us whether something is a store in a more direct fashion. I think it might be .subscribable?

Can you also add a test for this in test/runtime/samples?

[skippednote]

@Conduitry Thanks for the feedback. Instead of check the variable name I'm checking if it is subscribable. I have added a test as suggested.