fix yieldThread memory leak

helpers.ts

@@ -1,10 +1,15 @@
 import { verifiedSymbol, type Event, type Nostr, VerifiedEvent } from './core.ts'
 
 export async function yieldThread() {
-  return new Promise(resolve => {
+  return new Promise<void>(resolve => {
     const ch = new MessageChannel()
+    const handler = () => {
+      // @ts-ignore (typescript thinks this property should be called `removeListener`, but in fact it's `removeEventListener`)
+      ch.port1.removeEventListener('message', handler)
+      resolve()
+    }
     // @ts-ignore (typescript thinks this property should be called `addListener`, but in fact it's `addEventListener`)
-    ch.port1.addEventListener('message', resolve)
+    ch.port1.addEventListener('message', handler)
     ch.port2.postMessage(0)
     ch.port1.start()
   })