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.