LibWeb: Port the WindowOrWorkerGlobalScopeMixin timer handler to String

This will make porting to IDL a bit cleaner.

Userland/Libraries/LibWeb/HTML/Window.cpp

@@ -1370,7 +1370,7 @@ static JS::ThrowCompletionOr<TimerHandler> make_timer_handler(JS::VM& vm, JS::Va
 {
     if (handler.is_function())
         return JS::make_handle(vm.heap().allocate_without_realm<WebIDL::CallbackType>(handler.as_function(), incumbent_settings_object()));
-    return TRY(handler.to_deprecated_string(vm));
+    return TRY(handler.to_string(vm));
 }
 
 // https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#dom-settimeout

Userland/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.cpp

@@ -211,7 +211,7 @@ i32 WindowOrWorkerGlobalScopeMixin::run_timer_initialization_steps(TimerHandler
                     report_exception(result, this_impl().realm());
             },
             // 3. Otherwise:
-            [&](DeprecatedString const& source) {
+            [&](String const& source) {
                 // 1. Assert: handler is a string.
                 // FIXME: 2. Perform HostEnsureCanCompileStrings(callerRealm, calleeRealm). If this throws an exception, catch it, report the exception, and abort these steps.
 

Userland/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.h

@@ -18,7 +18,7 @@
 namespace Web::HTML {
 
 // https://html.spec.whatwg.org/#timerhandler
-using TimerHandler = Variant<JS::Handle<WebIDL::CallbackType>, DeprecatedString>;
+using TimerHandler = Variant<JS::Handle<WebIDL::CallbackType>, String>;
 
 // https://html.spec.whatwg.org/multipage/webappapis.html#windoworworkerglobalscope
 class WindowOrWorkerGlobalScopeMixin {