LibWeb: Post all MessagePort messages over their LocalSockets

This is to allow future changes to do cross-process MessagePorts in an
implementation-agnostic way. Add some tests for this behavior.

Delivering messages that were posted to a MessagePort just before it was
transferred is not yet implemented still.

Tests/LibWeb/Text/input/Messaging/Messaging-post-channel-over-channel.html

@@ -0,0 +1,32 @@
+<script src="../include.js"></script>
+<script>
+    asyncTest(done => {
+        let channel = new MessageChannel();
+
+        channel.port1.onmessage = (event) => {
+            println("Port1: " + JSON.stringify(event.data));
+            if (event.ports.length > 0) {
+                event.ports[0].postMessage("Hello from the transferred port");
+                return;
+            }
+            channel.port1.postMessage(event.data);
+        };
+
+        channel.port2.onmessage = (event) => {
+            println("Port2: " + JSON.stringify(event.data));
+            if (event.data === "DONE") {
+                done();
+            }
+        };
+
+        let channel2 = new MessageChannel();
+
+        channel2.port2.onmessage = (event) => {
+            println("Port3: " + JSON.stringify(event.data))
+        }
+
+        channel.port2.postMessage("Hello");
+        channel.port2.postMessage({ foo: channel2.port1 }, { transfer: [channel2.port1] });
+        channel.port2.postMessage("DONE");
+    });
+</script>