fix useWebSocketImplementation so it works with pool on nodejs esm.

pool.ts

@@ -1,9 +1,21 @@
+/* global WebSocket */
+
 import { verifyEvent } from './pure.ts'
 import { AbstractSimplePool } from './abstract-pool.ts'
 
+var _WebSocket: typeof WebSocket
+
+try {
+  _WebSocket = WebSocket
+} catch {}
+
+export function useWebSocketImplementation(websocketImplementation: any) {
+  _WebSocket = websocketImplementation
+}
+
 export class SimplePool extends AbstractSimplePool {
   constructor() {
-    super({ verifyEvent })
+    super({ verifyEvent, websocketImplementation: _WebSocket })
   }
 }