LibCore: Add a standard path for application data

Author: https://github.com/trflynn89
Commit: 49d74ee288
Pull-request: https://github.com/SerenityOS/serenity/pull/16358
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibCore/StandardPaths.h

@@ -18,6 +18,7 @@ public:
     static DeprecatedString downloads_directory();
     static DeprecatedString tempfile_directory();
     static DeprecatedString config_directory();
+    static DeprecatedString data_directory();
 };
 
 }