Party.Server — New API for a programmable primitive

The new Party.Server API simplifies developing real-time, collaborative, multiplayer applications using PartyKit. Parties are globally distributed, stateful, programmable, and on-demand web servers powered by Cloudflare Durable Objects. The API includes classes for server instances, lifecycle methods such as onStart, onConnect, onClose, onError, and onMessage, request handlers like onRequest and static onBeforeRequest, onBeforeConnect, and onFetch methods. Additionally, the API supports hibernation for cost-effective scaling and better connection management with tagged connections. The naming has been updated to clarify the distinction between "parties" and "rooms".