chore: better store subscriptions

[Rich-Harris]

Noticed a few opportunities for improvement with our store logic, figured I'd quickly address them even though it's low priority. First, the compiler output is improved. Before:

const $$subscriptions = {};

$.unsubscribe_on_destroy($$subscriptions);

const $count = () => $.store_get(count, "$count", $$subscriptions);
let count = writable(0);

After (we can create the subscriptions and setup the teardown in a single step):

const $$stores = $.setup_stores();
const $count = () => $.store_get(count, "$count", $$stores);
let count = writable(0);

The runtime also gets a bit smaller. We can use the existing teardown function which is more efficient than creating a new on_destroy function, and we can simplify store_get — last_value was never written to, so whatever that logic was attempting to do, it wasn't doing it.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 24d5e7b

The changes in this PR will be included in the next version bump.

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[dummdidumm]

This was in response to a real bug I investigated at some point. I think the fix was gradually removed over time (there was more logic there before) so that it's unused now. Let me see if I can dig up the PR and related issue to see if something like this is still necessary.

[dummdidumm]

It was from https://github.com/sveltejs/svelte-octane/commit/6ebe7118c8fe3f45d0b9787d1e9c92cc7e27d62d, but without any reference - I vaguely remember that it occured while testing the code out on some Svelte site and noticing the bug. But this error can't happen anymore now because back then the source value was set back to UNINITIALIZED which is no longer the case now - so all good.