Update CONTRIBUTING.md to reflect pnpm and vitest usage

[AJLemos]

While following the CONTRIBUTING.md guide I ran into a few issues running yarn commands until I took a look at package.json and saw the repo now uses pnpm (great choice btw).

• The repo was updated to use pnpm here: chore: switch to pnpm from yarn #611
• The repo was updated to use vitest here: Rewrite unit tests - vitest + RTL #604

The E2E command still needs to be updated but I haven't worked through installing and running cypress yet so I wasn't sure what the correct documentation there should be yet.

[yoavniran]

Thanks @AJLemos !
I totally forgot about updating it so thank you for the PR 😆

About E2E, its a good point, I should add that too. It's a little involved and I think there are a couple of specs that dont pass without special flags. I need to sit myself down and document it thoroughly.

[github-actions]

📦 Bundle Size Report

name	size	max	success
ui-core-chunked	15.96KB	16.6KB	🟢
core	10.59KB	10.74KB	🟢
ui-core	13.54KB	14.65KB	🟢
all	23.76KB	24.41KB	🟢