extends... in jsconfig causes warnings in editors

Describe the bug

VSCode gives type warnings in vanilla js

Removing the following from jsconfig.json fixes this:

"extends": "./.svelte-kit/tsconfig.json"

This shouldn’t happen if you are not using typescript.

Reproduction

Create a brand new SvelteKit project, skeleton, no typescript. Add following to index.svelte

<script>
    import { session } from '$app/stores'
</script>

{$session.lang}

VSCode will now mark lang with a warning/error saying Property 'lang' lang does not exist on type 'Session'

Logs

No response

System Info

System:
    OS: Windows 10 10.0.19044
    CPU: (12) x64 Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz
    Memory: 13.28 GB / 31.71 GB
  Binaries:
    Node: 16.14.0 - C:\Program Files\nodejs\node.EXE
    npm: 8.3.1 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.1266.0), Chromium (99.0.1150.46)
    Internet Explorer: 11.0.19041.1566
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.33
    @sveltejs/kit: next => 1.0.0-next.302
    svelte: ^3.44.0 => 3.46.4

Severity

annoyance

Additional Information

No response

[Rich-Harris]

Cross-posting from #4495:

---

I almost wonder if we need to have two separate prompts when setting up a project, where if you don’t opt in to TypeScript-the-language you can still opt in to typechecking:

? Use TypeScript? › Yes

? Use TypeScript? › No
? Add typechecking for JavaScript? › No / Yes

Personally I would choose No and Yes, but I don’t know if I’m in a sufficiently small minority that that would be annoying for most non-TypeScript users.

[Rich-Harris]

We can close this now, as jsconfig.json is only generated if you choose ‘Type-checked JavaScript’ when scaffolding the project, and the extended config doesn’t include checkJs (it’s part of the default jsconfig.json if you opt in)