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/Bindings/MainThreadVM.cpp

@@ -366,4 +366,28 @@ void queue_mutation_observer_microtask(DOM::Document& document)
     });
 }
 
+// https://html.spec.whatwg.org/multipage/webappapis.html#creating-a-new-javascript-realm
+NonnullOwnPtr<JS::ExecutionContext> create_a_new_javascript_realm(JS::VM& vm, Function<JS::Value(JS::Realm&)> create_global_object, Function<JS::Value(JS::Realm&)> create_global_this_value)
+{
+    // 1. Perform InitializeHostDefinedRealm() with the provided customizations for creating the global object and the global this binding.
+    // 2. Let realm execution context be the running JavaScript execution context.
+    auto realm_execution_context = MUST(JS::Realm::initialize_host_defined_realm(vm, move(create_global_object), move(create_global_this_value)));
+
+    // 3. Remove realm execution context from the JavaScript execution context stack.
+    vm.execution_context_stack().remove_first_matching([&realm_execution_context](auto* execution_context) {
+        return execution_context == realm_execution_context.ptr();
+    });
+
+    // NO-OP: 4. Let realm be realm execution context's Realm component.
+    // NO-OP: 5. Set realm's agent to agent.
+
+    // FIXME: 6. If agent's agent cluster's cross-origin isolation mode is "none", then:
+    //          1. Let global be realm's global object.
+    //          2. Let status be ! global.[[Delete]]("SharedArrayBuffer").
+    //          3. Assert: status is true.
+
+    // 7. Return realm execution context.
+    return realm_execution_context;
+}
+
 }