fix: prevent crash when accessing TextDecoder (#684)

The TextDecoder object was added on the global object in Node.js
v11.0.0, so older versions would throw:

> ReferenceError: TextDecoder is not defined

Reference: https://nodejs.org/api/util.html#new-textdecoderencoding-options

lib/server.ts

@@ -16,6 +16,7 @@ import type { CookieSerializeOptions } from "cookie";
 import type { CorsOptions, CorsOptionsDelegate } from "cors";
 import type { Duplex } from "stream";
 import { WebTransport } from "./transports/webtransport";
+import { TextDecoder } from "util";
 
 const debug = debugModule("engine");