LibWeb: Add NavigateEvent, the main event of the Navigation API

This event is the star of the show, and the main way that web content
can react to either programmatic or user-initiated navigation.

All of the fun algorithms will have to come later though.

Userland/Libraries/LibWeb/Forward.h

@@ -414,8 +414,10 @@ class MimeType;
 class MimeTypeArray;
 class Navigable;
 class NavigableContainer;
+class NavigateEvent;
 class Navigation;
 class NavigationCurrentEntryChangeEvent;
+class NavigationDestination;
 class NavigationHistoryEntry;
 class NavigationTransition;
 class Navigator;