LibWeb/UIEvents: Implement InputEvent

Author: https://github.com/jamierocks
Commit: 9fce70069d
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/701
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/tcl3

Userland/Libraries/LibWeb/Forward.h

@@ -726,6 +726,7 @@ class SVGTitleElement;
 }
 
 namespace Web::UIEvents {
+class InputEvent;
 class KeyboardEvent;
 class MouseEvent;
 class PointerEvent;