feat(vitest): add jest-like `Mock` type

[hi-ogawa]

Description

• closes https://github.com/vitest-dev/vitest/issues/4723
• related https://github.com/vitest-dev/vitest/pull/4784

This is a non breaking version of https://github.com/vitest-dev/vitest/pull/4784 by exposing new jest-like Mock type under a separate namespace based on the suggestion https://github.com/vitest-dev/vitest/pull/4784#issuecomment-1871287808.

todo

• is it okay to use __Mock to indicate these are internal things?
• naming? (currently import { Mocks } from "vitest" and Mocks.Mock)
• docs
  • https://vitest.dev/guide/migration.html#types

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• [x] It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• [x] Ideally, include a test that fails without this PR but passes with it.
• [ ] Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• [ ] Run the tests with pnpm test:ci.

Documentation

• [ ] If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• [x] Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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[sheremet-va]

Closing in favor of #4784