Add pnpm compatibility

[julienloizelet]

From version 2.2.0 onwards, we can use npm instead of yarn.

We could also work to add pnpm alternative.

To begin with, we need to install it in the Playwright container.

Probably something like the ddev viteserve add-on approach will work well.

Then, we would have to update the playwright-install and playwright-init command accordingly.

And add some tests if possible.