GNOME Shell uses GJS, not Seed. There are some differences among both of them, and it might explain why you are having problems.
With GOBject Introspection you can read the documentation for the original library and adapt the signature of every method/function to the language you are using.
In your particular case, for files, networking, etc. you might want to check GIO. You can check the Platform Overview to check what else is available. You also can check some examples (not extensions, though) in JavaScript.
At last but not least, very soon a website with extension will be launched.