stream: prevent unhandled rejection in Broadcast.from() pump

`Broadcast.from()` used a fire-and-forget async IIFE to pump data
from source to writer. While the catch block routes all errors
to `writer.fail()` which never rejects, the IIFE's promise was
unguarded against future invariant changes.

Extract the IIFE to a named pump function and attach a no-op
rejection handler via `PromisePrototypeThen`.

Author: jasnell

lib/internal/streams/iter/broadcast.js

@@ -13,6 +13,7 @@ const {
   ArrayPrototypeSlice,
   MathMax,
   Promise,
+  PromisePrototypeThen,
   PromiseResolve,
   SafeSet,
   String,
@@ -679,7 +680,7 @@ const Broadcast = {
     const result = broadcast(options);
     const signal = options?.signal;
 
-    (async () => {
+    const pump = async () => {
       const w = result.writer;
       try {
         if (isAsyncIterable(input)) {
@@ -721,7 +722,8 @@ const Broadcast = {
           await w.fail(err);
         }
       }
-    })();
+    };
+    PromisePrototypeThen(pump(), undefined, () => {});
 
     return result;
   },