New JSON reporter

Clear and concise description of the problem

The current JSON reporter is a jest-compatible reporter made exclusively to support tools that require that format. Vitest doesn't store tests in the same format internally, so it doesn't make sense to expose it that way. We need a separate json reporter.

Suggested solution

1. Alias json reporter to jest-json, print a warning if used
2. Add a new json reporter that uses Vitest tasks

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.

Start by reading the existing json and jest-json reporter implementations and the surrounding reporter tests. Determine whether the issue should first warn while aliasing json to jest-json, or add a separate reporter based on Vitest tasks; done means the chosen behavior is implemented and covered by reporter tests.