feat(vitest): add vitest cleanup export

[mcous]

Overview

This is the first PR in a series to add multiple Svelte versions and test runners to our CI test matrix.

1. feat(vitest): add vitest cleanup export #294 <-- you are here
2. ci: test against svelte v3 and v4 #295
3. TODO add multiple Vitest environments to CI matrix
4. TODO add Jest to CI matrix

Change log

This PR concentrates on getting Vitest fully working, since there are a few issues with the setup

• Add @testing-library/svelte/vitest import, inspired by @testing-library/jest-dom/vitest to run auto-cleanup when using Vitest in non-global mode
  • Will require a docs update at some point
• Upgrade testing-library/jest-dom so we can use @testing-library/jest-dom/vitest
• Fix test scripts for watch mode and snapshot updates, which were still written with jest arguments
• Fix Vitest coverage reports, which were not being generated in CI

Review requests

I'm keeping this PR in draft mode until I can verify that the @testing-library/svelte/vitest import works in a real project. Importing vitest in library code has given me issues before, so I want to verify before this is merged/released. It definitely works for our own test suite, though! Verified, works like a charm

[mcous]

Renamed / moved this file to avoid confusion with the src/vitest.js entrypoint

[github-actions]

🎉 This PR is included in version 4.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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