fix(types): add explicit extension for ESM compatibility

[jgoux]

Hello 👋

When consuming vitest/config from my own project which is using ESM ("type": "module"), defineConfig is seen as any.

I think this is because vitest is itself using ESM, so the types definition files have to use explicit paths when importing/exporting local files.

I think the issue was never detected before because the types definition files outside of config.d.ts and node.d.ts are all flattened and don't import/export local files.

I already encountered this issue here: tinyhttp/tinyhttp#359

What's a bit strange in your case is that TypeScript seems to see the types, but @typescript-eslint can't (it relies in TypeScript itself for types resolution 🤔):

Anyway, I think if you are declaring your module as ESM and you are writing it in TypeScript, it's a best practice (or even a requirement?) to use explicit paths in the import/export statements for local files.

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