feat!: always run tests inside worker

[antfu]

Breaking Changes

• threads: false now runs all tests in the same worker, instead of the main process
• the isolate option is now orthogonal to threads. By solely threads: false, the context will still be destroyed after each run. Combining threads: false and isolate: false to get the previous behavior of threads: false that context will leak across tests.
• env options will now only affect the tests but not the main process's process.env

This PR is for the preparation of supporting multiple projects #256

[netlify]

✅ Deploy Preview for vitest-dev ready!

Name	Link
🔨 Latest commit	e073922
🔍 Latest deploy log	https://app.netlify.com/sites/vitest-dev/deploys/6279de60e2ada80009d3e9d4
😎 Deploy Preview	https://deploy-preview-1278--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[sheremet-va]

I think --no-threads should always run all files in a single worker, otherwise there is no point in --no-threads. It's literally the same as running --threads - no alternative. And the name becomes misleading.

I think --isolate should just not affect --no-threads. Maybe in the future we might use vm.runInContext for isolation in --no-threads.