Declare & force consistent package manager

[Xevion]

We should make sure the repository has a defined package manager that all members are working with, for compatibility purposes. Node version & package manager version should be checked automatically. Whether this is yarn or pnpm it doesn't matter, as long as we're consistent.

• https://github.com/pnpm/only-allow
• https://docs.npmjs.com/cli/v10/configuring-npm/package-json#engines

@Lermatroid As far as I know, you opened a new branch recently and made pnpm the new package manager.