test: migrate to vitest and colocate tests

[gnapse]

This migrates the repo from Jest to Vitest, adding a jsdom-based Vitest config and updating the test scripts and dependencies.

It also removes the root __test__ unit-test layout by colocating the parser and interpolate suites with the modules they cover.

The built-package smoke and ESM/CJS type-consumer checks are kept, but moved into package-checks/ because they validate publish-time package contracts rather than source-level behavior.

Reference

• Modernizing @doist/react-interpolate
• Follow-up to
  • feat!: convert react-interpolate to TypeScript #55
  • feat!: migrate @doist/react-interpolate to tsdown #56
  • chore: replace prettier and eslint with oxfmt and oxlint #57

[doistbot]

This PR successfully migrates the testing framework from Jest to Vitest and improves project organization by colocating unit tests with their respective modules. These updates streamline the test configuration and nicely separate source-level behavior from package-level publish checks. A few adjustments have been noted regarding a broken oxlint override caused by the file reorganization, alongside some suggested refinements to test helpers, React rendering patterns, and the placement of global cleanup hooks to optimize test execution.

_{Share Feedback • Review Logs}