Export FixtureFn and related types

Clear and concise description of the problem

I'd like to use the FixtureFn type so I can create a typesafe fixture factory. I can contribute a PR, but need a little bit of guidance.

Suggested solution

I think it's just a matter of adding some exports of the types from @vitest/runner to vitest, but I'm not sure where exactly it should be exported. I'm not super familiar with the codebase.

Alternative

If I wasn't using pnpm I guess I could just import them directly from @vitest/runner (or I'd have to install it directly), I can also copy-paste them into my codebase, but obviously I don't like this solution.

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start by tracing how types from @vitest/runner are exposed through the vitest package entry points. Identify the exports needed for FixtureFn and its related types, then verify they can be imported from vitest without directly installing or importing @vitest/runner. The issue does not name specific files or tests, so repository guidance will be needed before implementation.