feat: Refactor server implementation to support Bun and Node environments

- Introduced `ServerNode` and `BunServer` classes to handle server logic for Node and Bun respectively.
- Updated `App` class to initialize the appropriate server based on the runtime environment.
- Enhanced `parseBody` function to handle request body parsing for both environments.
- Modified WebSocket handling to support Bun's WebSocket upgrade mechanism.
- Improved error handling and response structure across the server implementation.
- Added support for custom middleware in the server's request handling.
- Refactored server base functionality into `ServerBase` for better code organization.
- Updated type definitions to reflect changes in server options and listener handling.
- Added a new demo for testing the server functionality with various request types.

src/router-simple.ts

@@ -62,6 +62,7 @@ export class SimpleRouter {
   }
   isSse(req: Req) {
     const { headers } = req;
+    if (!headers) return false;
     if (headers['accept'] && headers['accept'].includes('text/event-stream')) {
       return true;
     }