chore: treeshakeable store subs

[Rich-Harris]

In #10503 I removed the store_sub_exist mechanism for omitting store subscriptions for directive names, because it's not a great use of our complexity budget to optimise such a niche case while leaving more common uses (event handlers, effects, etc) unoptimised.

This PR proposes a more universal approach — instead of trying to do the static analysis ourselves, we leave it to tools like Rollup. With this PR, a component like this...

<script>
  import { writable } from 'svelte/store';

  const thing = writable();

  function foo() {
    bar();
  }

  function bar() {
    console.log($thing);
  }

  $effect(() => {
    console.log($thing);
  });
</script>

<button onclick={foo}>click me</button>

...gets turned into this...

export default function App($$payload, $$props) {
  $.push(true);

-  const $$store_subs = {};
+  var $$store_subs;
  const thing = writable();

  function foo() {
    bar();
  }

  function bar() {
-    console.log($.store_get($$store_subs, "$thing", thing));
+    console.log($.store_get($$store_subs ??= {}, "$thing", thing));
  }

  $$payload.out += `<button>click me</button>`;
-  $.unsubscribe_stores($$store_subs);
+  if ($$store_subs) $.unsubscribe_stores($$store_subs);
  $.pop();
}

...which Rollup turns into this (not sure why it can't treeshake writable() away, but that's a separate issue):

function App($$payload, $$props) {
  $.push(true);
  writable();

  $$payload.out += `<button>click me</button>`;
  $.pop();
}

The trade-off is that in the (perhaps more common?) case that a store value contributes to SSR, each store_get reference gets a ??= {} added, and the unsubscribe_stores(...) call now sits inside a if ($$store_subs) block.

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: df5a764

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

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