feat(vitest): modernize AE and support Vitest 4

[IlCallo]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• New test runner
• Documentation
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

If you are adding a new test runner, have you...? (check all)

• Created an issue first?
• Registered it in /packages/base/runners.json?
• Added it to /README.md?
• Included one test that runs baseline.spec.vue?
• Added and updated documentation?
• Included a recipe folder with properly building quasar project?

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch and not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on Windows
• It's been tested on Linux
• It's been tested on MacOS
• Any necessary documentation has been added or updated in the docs (for faster update click on "Suggest an edit on GitHub" at bottom of page) or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[ReneBrandt]

Hi, I see you are still busy on this, just wanted to let you know this change is still very much desired from at least me :)

[q00u]

I'm bumping up against this too. Not having proper unit tests working makes me very uncomfortable, haha.

I'm on linux and would be happy to test it, but I don't know how to install a quasar app extension from a branch package (it doesn't appear to work the same as pnpm)

[joepvdstaaij]

What is the progress on this? Early stages, almost done?

[JochemVanIterson]

I'm also curious about the state of this PR. I've currently disabled vitest in my Quasar projects as it caused me to not be able to update a vulnerable package. I presume that with the new update to @quasar/app-vite v3 more issues will surface and incompatibility to widen further.

Is there anything we as a community can do to help? Maybe you could provide a test plan on how to test an AE and this specific feature?

[marvin-wtt]

For my project, I removed quasar testing. I only used installQuasarPlugin anyway, so I just copied the file