feat: add $app/state module

[Rich-Harris]

Closes #10761

This adds a new $app/state module, which is basically the same as the existing $app/stores module but using Svelte 5 state. (Needless to say, it only works with Svelte 5.)

This brings many benefits:

• no more weird $ prefix! you basically don't need to learn stores any more, except for dealing with legacy code
• no more weird timing stuff. Because store subscribe callbacks are synchronous, it's possible to get into a weird state where e.g. $page.url updates before navigation occurs. We jump through weird hoops (the notifiable_store stuff) to prevent this from causing bugs in people's apps
• as a corollary to the above, once we can get rid of $app/stores (which I'd like to do in SvelteKit 3), we'll no longer need to awkwardly pass stuff into the root component
• properties are fine-grained. an update to page.data won't cause something that only cares about an unrelated property like page.status to re-run

Related PRs:

• migrate script: feat: add app-state migration cli#354
• tutorial: App state tutorial svelte.dev#1016

TODO:

• figure out how to make it work for Svelte 4 apps
• do the same thing for navigating and updated
• create a sv migrate task to automate the migration
• use said migration to update all the tests in this repo
• deprecate everything in $app/stores (the sooner we do this, the less disruptive it'll be when we remove them)
• update the tutorial
• update the docs

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13140-svelte.vercel.app/

this is an automated message

[dummdidumm]

Some thoughts on the migration:

• either we do this very simple string-based. This will likely work for 95% or more of all use cases, because of the unique constraints that work in our favor here:
  • check if there's a page import from $app/store
  • make sure we're in runes mode by searching for clear indicators like $props() or $state( or $derived( or $effect(
  • make sure that page as-is is not used
  • if not, then replace all $page with page
• or we do it in Svelte core as part of the migrate function
  • add a new parameter migration to specify which migration specifically you want (default to "do all of them")
  • if migration: 'page store' then do the migration for the page store specifically
  • we can make use of more elaborate static analysis here; since we now support TypeScript it should run without issues for basically everyone