WebContent: Implement `$0` special variable in Browser JS Console

`$0` is a helpful variable in other browsers' JS consoles, which points
to whichever DOM Node is currently selected in the DOM Inspector. And
now we have it too! :^)

Userland/Services/WebContent/ConsoleGlobalObject.cpp

@@ -5,6 +5,8 @@
  */
 
 #include "ConsoleGlobalObject.h"
+#include <LibWeb/Bindings/NodeWrapper.h>
+#include <LibWeb/Bindings/NodeWrapperFactory.h>
 #include <LibWeb/Bindings/WindowObject.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Window.h>
@@ -20,6 +22,14 @@ ConsoleGlobalObject::~ConsoleGlobalObject()
 {
 }
 
+void ConsoleGlobalObject::initialize_global_object()
+{
+    Base::initialize_global_object();
+
+    // $0 magic variable
+    define_native_accessor("$0", inspected_node_getter, nullptr, 0);
+}
+
 void ConsoleGlobalObject::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);
@@ -87,4 +97,24 @@ JS::MarkedValueList ConsoleGlobalObject::internal_own_property_keys() const
     return m_window_object->internal_own_property_keys();
 }
 
+JS_DEFINE_NATIVE_GETTER(ConsoleGlobalObject::inspected_node_getter)
+{
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return JS::js_null();
+
+    if (!is<ConsoleGlobalObject>(this_object)) {
+        vm.throw_exception<JS::TypeError>(global_object, JS::ErrorType::NotA, "ConsoleGlobalObject");
+        return {};
+    }
+
+    auto console_global_object = static_cast<ConsoleGlobalObject*>(this_object);
+    auto& window = console_global_object->m_window_object->impl();
+    auto* inspected_node = window.document().inspected_node();
+    if (!inspected_node)
+        return JS::js_undefined();
+
+    return Web::Bindings::wrap(global_object, *inspected_node);
+}
+
 }

Userland/Services/WebContent/ConsoleGlobalObject.h

@@ -34,9 +34,14 @@ public:
     virtual bool internal_delete(JS::PropertyName const& name) override;
     virtual JS::MarkedValueList internal_own_property_keys() const override;
 
+    virtual void initialize_global_object() override;
+
 private:
     virtual void visit_edges(Visitor&) override;
 
+    // Because $0 is not a nice C++ function name
+    JS_DECLARE_NATIVE_GETTER(inspected_node_getter);
+
     Web::Bindings::WindowObject* m_window_object;
 };