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/MessageChannel.idl

@@ -0,0 +1,8 @@
+interface MessageChannel {
+
+    constructor();
+
+    readonly attribute MessagePort port1;
+    readonly attribute MessagePort port2;
+
+};