Release v6.0.0

• (@krobipd) Fixed: a command that cannot be spawned (missing executable, missing cwd) no longer terminates the whole test process with an unhandled 'error' event - executeCommand now resolves with the error instead
• (@mcm1957) Add validation to fail if a "prepare" script is defined in the "scripts" section of package.json
• (@GermanBluefox) Breaking: Dropped support for Node.js 18 and 20