LibWeb: Start implementing "create and initialize a Document" from HTML

The way we've been creating DOM::Document has been pretty far from what
the spec tells us to do, and this is a first big step towards getting us
closer to spec.

The new Document::create_and_initialize() is called by FrameLoader after
loading a "text/html" resource.

We create the JS Realm and the Window object when creating the Document
(previously, we'd do it on first access to Document::interpreter().)

The realm execution context is owned by the Environment Settings Object.

Userland/Libraries/LibWeb/Forward.h

@@ -202,6 +202,8 @@ class BrowsingContextContainer;
 class CanvasRenderingContext2D;
 class ClassicScript;
 class CloseEvent;
+struct CrossOriginOpenerPolicy;
+struct CrossOriginOpenerPolicyEnforcementResult;
 class DOMParser;
 class DOMStringMap;
 struct Environment;
@@ -287,11 +289,13 @@ class ImageData;
 class MessageChannel;
 class MessageEvent;
 class MessagePort;
+struct NavigationParams;
 class Origin;
 class PageTransitionEvent;
 struct PolicyContainer;
 class PromiseRejectionEvent;
 class WorkerDebugConsoleClient;
+struct SandboxingFlagSet;
 class Storage;
 class SubmitEvent;
 class TextMetrics;