chore: add cross-env dependency to examples/playwright

[xai]

What it does

This adds cross-env as dev dependency and adjusts the scripts in examples/playwright accordingly. This is a way to enable running the playwright tests on Windows machines.

Contributed on behalf of STMicroelectronics

How to test

1. Clone Theia on a Windows machine and checkout this branch
2. Build browser and electron examples
3. Build the playwright tests: cd examples/playwright; yarn; yarn build
4. Run the browser tests: yarn ui-tests

Follow-ups

We checked the status of the cross-env package and found that it is in maintenance mode. The authors of cross-env consider the development finished and fix only severe bugs and keep it up-to-date along with nodejs. The most recent commit in cross-env is 4 years old.

Please consider this and the implied additional threats regarding supply chain attacks that adding cross-env as a dev dependency might introduce into Theia when reviewing this PR.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[planger]

#12405 is related and possibly already fixed and enabled by this PR.