LibWeb+Ladybird+Userland: Port window.[alert,confirm,prompt] to String

LibGUI and WebDriver (read: JSON) API boundaries use DeprecatedString,
so that is as far as these changes can reach.

The one change which isn't just a DeprecatedString to String replacement
is handling the "null" prompt response. We previously checked for the
null DeprecatedString, whereas we now represent this as an empty
Optional<String>.

Userland/Services/WebContent/WebContentServer.ipc

@@ -70,5 +70,5 @@ endpoint WebContentServer
 
     alert_closed() =|
     confirm_closed(bool accepted) =|
-    prompt_closed(DeprecatedString response) =|
+    prompt_closed(Optional<String> response) =|
 }