wasm: fix setImmediate() implementation for Cloudflare Workers

This patch fixes setImmediate() implementation for Cloudflare Workers
in the wasm backend's js prelude script. Cloudflare Workers doesn't
support the MessageChannel API, and we use a setTimeout() based
fallback implementation in this case.

utils/jsffi/prelude.mjs

@@ -51,6 +51,10 @@ export class JSValManager {
 
 // The actual setImmediate() to be used. This is a ESM module top
 // level binding and doesn't pollute the globalThis namespace.
+//
+// To benchmark different setImmediate() implementations in the
+// browser, use https://github.com/jphpsf/setImmediate-shim-demo as a
+// starting point.
 const setImmediate = await (async () => {
   // node, bun, or other scripts might have set this up in the browser
   if (globalThis.setImmediate) {
@@ -67,30 +71,35 @@ const setImmediate = await (async () => {
     return (cb, ...args) => scheduler.postTask(() => cb(...args));
   }
 
-  // A simple & fast setImmediate() implementation for browsers. It's
-  // not a drop-in replacement for node.js setImmediate() because:
-  // 1. There's no clearImmediate(), and setImmediate() doesn't return
-  //    anything
-  // 2. There's no guarantee that callbacks scheduled by setImmediate()
-  //    are executed in the same order (in fact it's the opposite lol),
-  //    but you are never supposed to rely on this assumption anyway
-  class SetImmediate {
-    #fs = [];
-    #mc = new MessageChannel();
+  // Cloudflare workers doesn't support MessageChannel
+  if (globalThis.MessageChannel) {
+    // A simple & fast setImmediate() implementation for browsers. It's
+    // not a drop-in replacement for node.js setImmediate() because:
+    // 1. There's no clearImmediate(), and setImmediate() doesn't return
+    //    anything
+    // 2. There's no guarantee that callbacks scheduled by setImmediate()
+    //    are executed in the same order (in fact it's the opposite lol),
+    //    but you are never supposed to rely on this assumption anyway
+    class SetImmediate {
+      #fs = [];
+      #mc = new MessageChannel();
 
-    constructor() {
-      this.#mc.port1.addEventListener("message", () => {
-        this.#fs.pop()();
-      });
-      this.#mc.port1.start();
-    }
+      constructor() {
+        this.#mc.port1.addEventListener("message", () => {
+          this.#fs.pop()();
+        });
+        this.#mc.port1.start();
+      }
 
-    setImmediate(cb, ...args) {
-      this.#fs.push(() => cb(...args));
-      this.#mc.port2.postMessage(undefined);
+      setImmediate(cb, ...args) {
+        this.#fs.push(() => cb(...args));
+        this.#mc.port2.postMessage(undefined);
+      }
     }
+
+    const sm = new SetImmediate();
+    return (cb, ...args) => sm.setImmediate(cb, ...args);
   }
 
-  const sm = new SetImmediate();
-  return (cb, ...args) => sm.setImmediate(cb, ...args);
+  return (cb, ...args) => setTimeout(cb, 0, ...args);
 })();