Improve error message when a Svelte config is not found

[ykomatsu]

svelte-kit error message can be a bit confusing when both svelte.config.js and svelte.config.cjs are absent.
We need svelte.config.js , not svelte.config.cjs :

$ npm run dev
Cannot find module '/Users/ykoma/Repositories/kit-playground/svelte.config.cjs' imported from /Users/ykoma/Repositories/kit-playground/node_modules/@sveltejs/kit/dist/cli.js
...

After this PR:

$ npm run dev
Cannot find module '/Users/ykoma/Repositories/kit-playground/svelte.config.js' imported from /Users/ykoma/Repositories/kit-playground/node_modules/@sveltejs/kit/dist/cli.js
...

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🦋 Changeset detected

Latest commit: b70a118

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

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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[dummdidumm]

Please provide a changeset, other than that this looks good 👍

[Rich-Harris]

Thank you! This still isn't a great error message though — it should be a much more explicit 'Missing svelte.config.js', or perhaps

You need to create a svelte.config.js file — see https://kit.svelte.dev/docs#configuration

In fact, there's already some logic for that, it's just a) in a weird place that means it never actually runs, and b) overly complicated. We should just check to see if svelte.config.js or svelte.config.cjs exists, and if not throw a helpful error inside load_config

[Rich-Harris]

Thanks. Realised we can actually simplify this stuff quite a bit, because the checks for svelte.config.cjs and vite.config.js are from many months ago, and really aren't needed any more