add svelte-kit sync

[Rich-Harris]

Closes #4165. This adds a sync module responsible for writing stuff to outDir — generated/manifest.js, generated/root.svelte, runtime, types and tsconfig.json.

It also adds a svelte-kit sync command that runs when you first set up a new project, to ensure that types are set up and correct before you run typechecking.

I've been meaning to tidy up this corner of the codebase anyway, and this was a nice opportunity to do it.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

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• Ideally, include a test that fails without this PR but passes with it.

Tests

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

Changesets

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[changeset-bot]

🦋 Changeset detected

Latest commit: 800476b

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

This PR includes changesets to release 2 packages

Name	Type
create-svelte	Patch
@sveltejs/kit	Patch

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[benmccann]

Is this command cheap enough that we can run it whenever we want to run pnpm check? Or would it be helpful to also have a svelte-kit tsconfig command that just generates that file? (#4190 (comment))

[Rich-Harris]

For a large-ish app it takes about 100ms on my machine — it's basically free