LibWeb: Implement basic support for MessageChannel and MessagePort

This patch adds a basic initial implementation of these API's.

Since LibWeb currently doesn't support workers, this implementation of
messaging doesn't bother with serializing and deserializing messages.

Base/res/html/misc/message-channel.html

@@ -0,0 +1,7 @@
+<script>
+var channel = new MessageChannel();
+channel.port2.onmessage = function(event) {
+    console.log("Port2 received '" + event.data + "'");
+}
+channel.port1.postMessage("HELLO FRIEND");
+</script>

Base/res/html/misc/welcome.html

@@ -127,6 +127,7 @@
 
         <h2>JavaScript/Wasm</h2>
         <ul>
+            <li><a href="message-channel.html">MessageChannel</a></li>
             <li><a href="websocket.html">WebSocket API Test</a></li>
             <li><a href="cookie.html">document.cookie</a></li>
             <li><a href="event-bubbling-and-multiple-listeners.html">event bubbling and multiple listeners</a></li>