docs: add vitest intro to typescript/testing

[zxch3n]

I'm not sure whether a gif screenshot is too bothering. Let me know if you guys prefer code examples or static screenshots ;)

[kenshanta]

Is this PR still valid? 👀

[ntrogh]

Closing this PR as we need to rewrite the content for testing in TypeScript. Current general testing guidance is available here: https://code.visualstudio.com/docs/editor/testing