LibWeb: Implement Window.prompt()

Author: https://github.com/linusg
Commit: 5e07c27e25
Pull-request: https://github.com/SerenityOS/serenity/pull/5423
Reviewed-by: https://github.com/awesomekling

Userland/Services/WebContent/WebContentClient.ipc

@@ -16,4 +16,5 @@ endpoint WebContentClient = 90
     DidRequestLinkContextMenu(Gfx::IntPoint content_position, URL url, String target, unsigned modifiers) =|
     DidRequestAlert(String message) => ()
     DidRequestConfirm(String message) => (bool result)
+    DidRequestPrompt(String message, String default_) => (String response)
 }