Add @vue/compat for testing

What does this MR do and why?

Adds @vue/compat for tests to prepare for Vue 3 migration.

Largely copied from gitlab-ui!3087 (merged).

Test output: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/4222485195

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Update dependencies: make setup
3. Run Jest tests with both Vue 2 and @vue/compat: make jest-tests
4. Run @vue/compat tests only: yarn test:vue3

Expected behavior: all tests pass with both Vue 2 and @vue/compat.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.