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.

Userland/Libraries/LibWeb/HTML/MessagePort.idl

@@ -0,0 +1,10 @@
+interface MessagePort : EventTarget {
+
+    undefined postMessage(any message);
+    undefined start();
+    undefined close();
+
+    attribute EventHandler onmessage;
+    attribute EventHandler onmessageerror;
+
+};