chore: configure pnpm version for corepack

[Robbert]

Describe the pull request

Add code to support corepack enable

What

Support for corepack enable

Why

Your pnpm version is incompatible with "/Users/bobby/Sites/github.com/Amsterdam/design-system".

Expected version: ^9
Got: 8.15.8

This is happening because the package's manifest has an engines.pnpm field specified.
To fix this issue, install the required pnpm version globally.

How

Add packageManager property to package.json

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• corepack enable docs