LibWeb: Implement performance.now()

This patch introduces the HighResolutionTime namespace which is home to
the Performance object (exposed via window.performance)

performance.now() is currently the only function, and it returns the
number of milliseconds since the window object was constructed. :^)

Libraries/LibWeb/Forward.h

@@ -132,6 +132,10 @@ class HTMLVideoElement;
 class ImageData;
 }
 
+namespace Web::HighResolutionTime {
+class Performance;
+}
+
 namespace Web {
 class EventHandler;
 class Frame;
@@ -242,6 +246,7 @@ class ImageDataWrapper;
 class LocationObject;
 class MouseEventWrapper;
 class NodeWrapper;
+class PerformanceWrapper;
 class ScriptExecutionContext;
 class TextWrapper;
 class UIEventWrapper;