feat: hydratable and a more consistent remote functions model

[elliott-with-the-longest-name-on-github]

This PR makes a number of substantial changes to how remote queries work.

hydratable

Implementation-wise, it replaces our custom transport solution with hydratable for queries that are used during render -- this method of transport is more correct and prevents us from accidentally using stale data for queries in a small subset of cases. There's one breaking side effect of this: If you didn't render a query on the server, you can't render it during hydration.

It means this:

 <script>
  import { browser } from '$app/environment';
  const count = browser ? get_count() : null;
</script>

would throw during hydration because get_count tried to access cached data that did not exist. This is almost always an undesirable mistake: It means you introduced a waterfall on the client and blocked hydration. If you really do want this, you'd do something like this instead:

<script>
  import { onMount } from 'svelte';
  let count;
  onMount(() => {
    count = get_count();
  });
</script>

New rules around query usage

From now on, on the client, you can only access a query's data if that query is:

1. created in a tracking context (i.e. at the top level of a script block, in a derived, in an effect, etc -- basically somewhere Svelte's reactivity system can "see" the query)
2. the tracking context in which the query is created is still alive

It may be easier to illustrate when you can't create and use a query on the client:

• universal load
• event handlers
• the top level of modules

We're making this change because otherwise it's basically impossible to reliably cache queries across your app. However, for most use cases, this probably doesn't cause any pain:

• you can still use the methods that don't access query data anywhere: .refresh, .set, .withOverride are all available
• if you need to get a query's data in a non-reactive context, you can make a one-off request to get that query's data with query().run(), which will return a plain old Promise resolving to your data

Bugfixes

• queries are now wrapped in their own $effect.root when created. This prevents them from associating with their parent effect, making their usage more predictable when retrieved from the cache
• .refresh is now a noop if there is no cached instance of the query, meaning you won't have a useless query request in some circumstances
• caching / deduplicating should be more reliable, and it should be more obvious when you're trying to do something insane
• there should now be no cases where stale cached data is used post-hydration (thank you, hydratable!)

[changeset-bot]

🦋 Changeset detected

Latest commit: 95fdd23

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15533-svelte.vercel.app/

[Rich-Harris]

LGTM! nice work. only unresolved comment is about whether we want to merge #15555 or not. i leave it up to you