LibWeb: Add partially functioning Worker API

Add a partial implementation of HTML5 Worker API.
Messages can be sent from the inner context externally.

Userland/Libraries/LibWeb/Forward.h

@@ -215,11 +215,14 @@ class MessageEvent;
 class MessagePort;
 class PageTransitionEvent;
 class PromiseRejectionEvent;
+class WorkerDebugConsoleClient;
 class Storage;
 class SubmitEvent;
 class TextMetrics;
 class WebSocket;
 class WindowEnvironmentSettingsObject;
+class Worker;
+class WorkerEnvironmentSettingsObject;
 class WorkerGlobalScope;
 class WorkerLocation;
 class WorkerNavigator;
@@ -488,6 +491,7 @@ class URLSearchParamsWrapper;
 class URLWrapper;
 class WebSocketWrapper;
 class WindowObject;
+class WorkerWrapper;
 class WorkerGlobalScopeWrapper;
 class WorkerLocationWrapper;
 class WorkerNavigatorWrapper;