Everywhere: Use URL::about_XXX factory functions

Author: https://github.com/shannonbooth
Commit: 9072a7caef
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3586
Reviewed-by: https://github.com/tcl3 ✅

UI/Headless/main.cpp

@@ -63,7 +63,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
     WebView::platform_init();
 
-    auto app = Ladybird::Application::create(arguments, "about:newtab"sv);
+    auto app = Ladybird::Application::create(arguments, URL::about_newtab());
     TRY(app->launch_services());
 
     Core::ResourceImplementation::install(make<Core::ResourceImplementationFile>(MUST(String::from_byte_string(app->resources_folder))));