Add note to 3.0 migration guide about `svelte-kit sync` migration

[benmccann]

Describe the bug

In #13304, we are removing SvelteKit's preinstall script. Users should now ensure that their package.json contains:

"prepare": "svelte-kit sync"

In not present, users will see the following the first time they run Vite:

▲ [WARNING] Cannot find base config file "./.svelte-kit/tsconfig.json" [tsconfig.json]

    tsconfig.json:2:12:
      2 │   "extends": "./.svelte-kit/tsconfig.json",
        ╵              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This will be added to the template for new users, but existing projects should be migrated. We did this in a minor 2.x release in order to support the release of pnpm 10 since it only causes an extra message and not real harm. We did not automatically migrate existing projects. We had discussed printing a warning to notify users, but there was concern about false positives.

It would be good for the 3.x release notes to mention this and the migration tool to handle this if there is such a migration tool. While this change happened before 3.x, many users aren't going to check intermediate release notes and would not be aware they should make this change and so we should use the 3.x release to raise awareness.

Reproduction

n/a

Logs

System Info

n/a

Severity

annoyance

Additional Information

No response

[Sija]

It should be noted that this addition ought to land in the "scripts" part of the package.json. Additionally it's a prepare step/script, not preprepare.

[mamachanko]

I am still seeing the warning despite the presence of the prepare script.

❯ node --version
v20.10.0

❯ npm --version
10.2.3

❯ jq . package.json
{
  "name": "my-app",
  "private": true,
  "version": "0.0.1",
  "type": "module",
  "scripts": {
    "dev": "vite dev",
    "build": "vite build",
    "preview": "vite preview",
    "prepare": "svelte-kit sync || echo ''",
    "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
    "check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
    "format": "prettier --write .",
    "lint": "prettier --check .",
    "test:unit": "vitest",
    "test": "npm run test:unit -- --run"
  },
  "devDependencies": {
    "@sveltejs/adapter-auto": "^4.0.0",
    "@sveltejs/kit": "^2.16.0",
    "@sveltejs/vite-plugin-svelte": "^5.0.0",
    "autoprefixer": "^10.4.20",
    "prettier": "^3.4.2",
    "prettier-plugin-svelte": "^3.3.3",
    "prettier-plugin-tailwindcss": "^0.6.10",
    "svelte": "^5.0.0",
    "svelte-check": "^4.0.0",
    "tailwindcss": "^3.4.17",
    "typescript": "^5.0.0",
    "vite": "^6.0.0",
    "vitest": "^3.0.0"
  }
}

❯ npm run test

> my-app@0.0.1 test
> npm run test:unit -- --run


> my-app@0.0.1 test:unit
> vitest --run

▲ [WARNING] Cannot find base config file "./.svelte-kit/tsconfig.json" [tsconfig.json]

    ../tsconfig.json:2:12:
      2 │   "extends": "./.svelte-kit/tsconfig.json",
        ╵              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


 RUN  v3.0.4 /private/tmp/my-app/my-app

 ✓ src/demo.spec.ts (1 test) 3ms
   ✓ sum test > adds 1 + 2 to equal 3

 Test Files  1 passed (1)
      Tests  1 passed (1)
   Start at  13:52:06
   Duration  791ms (transform 21ms, setup 0ms, collect 26ms, tests 3ms, environment 0ms, prepare 119ms)

[benmccann]

Hmm. I have npm 11 and running npx sv create myapp created the file .svelte-kit/tsconfig.json

[mamachanko]

Hmm. I have npm 11 and running npx sv create myapp created the file .svelte-kit/tsconfig.json

@benmccann yeah, that's the thing. The file .svelte-kit/tsconfig.json was present but the warning remained. However, I am not able to reproduce the issue. Therefore, it's fair to ignore my report.