Update the package.json to specify the yarn version for corepack

[stof]

This makes it easier for contributors using corepack as their way of installing yarn (which is the recommended way nowadays)

[Kocal]

We should add packageManager config for our test apps aswell

[stof]

The packageManager in the main package.json breaks the CI for pnpm jobs because pnpm/action-setup detects conflicts between the packageManager and the specified version in the workflow. I need to investigate that. Maybe the solution is to change the workflow setup to tell pnpm/action-setup to look at the package.json of the test app instead, as that's where we use pnpm. I will investigate that.

We should add packageManager config for our test apps aswell

Indeed. That's a good catch. I'll do it.

[stof]

The yarn-based testing apps were already defining the packageManager field for corepack. I added it in all the pnpm testing apps, which makes it easier to have the same pnpm version locally than in CI (as the local setup does not care about the workflow definition).