[Bug]: SvelteKit mocks doesn't support the $app/state module

[JReinhold]

Describe the bug

SvelteKit 2.12.0 introduced a new built-in module, $app/state, intended for Svelte 5 usage.

• https://bsky.app/profile/svelte.dev/post/3ldhdcfc62k2k
• https://svelte.dev/docs/kit/$app-state

It functions similarly to the existing $app/stores module, but is based on runes instead of stores.

We have an (experimental) API that allows users to mock these SvelteKit-level built-in modules (originally contributed by @paoloricciuti), and we need to extend that to support $app/state as well. See:

• https://storybook.js.org/docs/get-started/frameworks/sveltekit#how-to-mock
• https://storybook.js.org/docs/get-started/frameworks/sveltekit#stores

Additional context

The mocks lives in https://github.com/storybookjs/storybook/blob/next/code/frameworks/sveltekit/src/mocks/app