Migrate from yarn to npm or pnpm

[Chocobozzz]

Describe the problem to be solved

yarn is not "useful" anymore as npm and pnpm have a lockfile now
We're stuck with yarn v1 that admins have to install as a global dependency. NixOS also has issues with yarn (see #6548 (comment))

Describe the solution you would like

We still need to investigate if we should move to npm or pnpm