Create a `diff` on the main thread

Clear and concise description of the problem

We already put prettified actual/received on every error that has a diff. We could just compare them on the main thread without needed to import the diff library (which takes ~15ms for every test file on M3 Macbook)

Suggested solution

Rewrite diff logic to be called on the main thread and save everyone some time when importing vitest. One issues I see at the moment is that this.utils.diff is a public API in extended matchers. I wonder if it would make sense to turn it into a CJS module that calls require inside utils.diff function to avoid breaking changes.

At the same time we could log a warning to return actual/expected instead of calling the diff manually (vitest creates a diff automatically then)

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 tracing the public this.utils.diff API in the extended matchers documentation and the current diff-library import path. Determine how main-thread comparison affects that API and verify that the diff library is no longer imported for every test file, while preserving or deliberately warning about the existing behavior.