LibWeb: Implement DataTransferItem.prototype.getAsString()

Author: https://github.com/trflynn89
Commit: 181ece4d9c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1163
Reviewed-by: https://github.com/tcl3 ✅

Userland/Libraries/LibWeb/HTML/DataTransferItem.h

@@ -25,6 +25,7 @@ public:
     String kind() const;
     String type() const;
 
+    void get_as_string(JS::GCPtr<WebIDL::CallbackType>) const;
     JS::GCPtr<FileAPI::File> get_as_file() const;
 
 private: