chore(package-manager): migrate to pnpm

[dargmuesli]

Just a demo for #52.

• demos pnpm usage as part of most Nuxt repos
• uses pnpm workspace to automatically initialize the project's types (playground) on installation
• pins development dependencies' versions, which makes it easier to point out issues with dependencies in development (can be updated using pnpm upgrade --interactive --latest or automatically using tools like renovate or dependabot)

Feel free to close if not wanted, or take parts of it :)