Configure Vitest to clear mocks automatically

[kentcdodds]

Summary

• add Vitest as a dev dependency and expose a dedicated bun run test:vitest script
• add vitest.config.ts with clearMocks: true and scoped file discovery
• add a focused proof test showing mock call history is reset between tests

Testing

• ✅ bun run test:vitest
• ✅ bunx prettier --check package.json vitest.config.ts vitest-tests/automatic-mock-clearing.test.ts
• ✅ bun run lint package.json vitest.config.ts vitest-tests/automatic-mock-clearing.test.ts