fix(types): allow Promise<void> return type for proxyRes callback (#1190)

The proxyRes callback type only allowed void return type, but when
using responseInterceptor, the handler is async and returns Promise<void>.

This fix updates the type to allow both void and Promise<void> return
types, matching the actual behavior of responseInterceptor.

Fixes #1151

Author: cyphercodes

src/types.ts

@@ -48,7 +48,7 @@ export interface OnProxyEvent<
     options: ProxyServerOptions,
     head: any,
   ) => void;
-  proxyRes?: (proxyRes: TReq, req: TReq, res: TRes) => void;
+  proxyRes?: (proxyRes: TReq, req: TReq, res: TRes) => void | Promise<void>;
   open?: (proxySocket: net.Socket) => void;
   close?: (proxyRes: TReq, proxySocket: net.Socket, proxyHead: any) => void;
   start?: (req: TReq, res: TRes, target: string | Partial<URL>) => void;