Fixing not running index.js tests

[subtle-byte]

Fixes #6330

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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[benmccann]

I wonder if the better fix might be to rename store/index.js to store/index.ts. It's the only one that's not TypeScript

[subtle-byte]

Fixed by #6293