Add developer documentation to core repo for debugging directories that use vitest

[hl662]

Vitest is awful when integrating with vscode's launch configuration, especially when it comes to debugging tests that are run through @vitest/browser. Vitest, unlike mocha, does not run only tests that use it.only across the whole folder, which also makes it a pain to selectively run tests the way our developers have done in the past.

One successful method of quickly running individual tests as well as debugging them, is to use VSCode's vitest explorer extension, and that's our recommendation. Although, it's not easy for developers to find that extension, and we don't explicitly provide that recommendation anywhere.

We should put that down in writing somewhere (perhaps CONTRIBUTING.md) to help new developers debug vitest-enabled folders in the core repo.