Replace mocha with vitest

[ssbarnea]

Describe the request

Considering the current state of mocha compared with vitest it would prove to be a wise move to replace use of mocha with vitest in extest as it would address lots of limitations of mocha and enable other powerful features that are provided by vitest.

Usage

[lppedd]

I'm not entirely sure how @djelinek implemented the test runner, but Mocha is still the only officially supported tool to run VS Code integration tests. Vitest does not work, Jest works with workarounds in older versions.

I would prefer to stick to Mocha, a single tool, as long as Microsoft doesn't come up with support for Vitest.

[ssbarnea]

We recently started to perform ui tests of vscode using pytest and a selenium container. This does not allow us to record coverage but seems to be stable. Also it does not allow testing of platform specific features, so is not really a replacement.