Introduce Vitest

[hl662]

Continuing off of the work from #6890

For core-bentley,core-geometry, core-common:

• Refactors all tests
• Uses vitest for coverage as well, needs explicit @vitest/coverage-istanbul (I'm open to switching to node's native code coverage v8 too, no opinions on either)
• Removed explicit chai, chai-as-promised, mocha, nyc dependencies

OneAtATime tests in core-bentley are skipped for now, haven't finished refactoring them.

Switching the integration tests to vitest is out of the scope of this PR, so just looking at the unit tests within each sub repo instead. We'll investigate this more after.

A couple more issues to circle back to later on as a result of this: #7200 #7201

[mergify[bot]]

This pull request is now in conflicts. Could you fix it @hl662? 🙏 To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

[aruniverse]

BTW, just pulled and tried out vitest, and the multithreading is so nice. All core-geometry tests done in 50s. Wow.

• @dassaf4

• just in case anyone comes back to this pr, dont want you to miss this comment :)