Our goal is to document all the APIs exposed by Scratch. We accept any contributions that work towards that goal.
Even if you aren't able to write types, please open issues to report missing or incorrect types.
In terms of code style, we aren't super picky. Just keep it consistent with whatever is already there.
For very complex types, please add a small test case in the tests directory. Presumably you already wrote some test code while writing the types, so just don't delete that once you're done.