allowExternal option for v8 coverage provider

[vojvodics]

Clear and concise description of the problem

Since the @vitest/coverage-c8 is removed in favor of @vitest/coverage-v8 (#3339) and coverage-v8 doesn't provide the allowExternal option this will break coverage reports when upgrading to the latest vitest version (and switching providers).

Suggested solution

Implement the allowExternal option (as suggested in the above-mentioned PR). I'd be happy to provide the implementation (LMK if you have any inputs/suggestions on how to do this)

Alternative

No response

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.

[AriPerkkio]

This should also be implemented to @vitest/coverage-istanbul. They both use the same test-exclude dependency which I think provides this in c8: https://github.com/bcoe/c8/blob/a13584d5be5259ebb6a00455d352c3e8b16006de/lib/report.js#L48-L52.

I'd be happy to provide the implementation

PR is welcome! Here is one PR for reference to show which files, tests and documentation needs changes when adding new coverage options: #3453.
Let me know if you need any help.

[vojvodics]

@AriPerkkio I drafted a PR for this, can you check it out? #3894

I actually would need help with writing good tests for this feature. I added some tests but I don't think they do what they should do. Please check out the comments in the PR 🙏