Add typescript types for the extension

[lxkuz]

As discussed on slack, I'd also suggest having a look at whether we can generate typescript types for the extension using something like https://github.com/OneOfOne/struct2ts or https://github.com/tkrajina/typescriptify-golang-structs. This would definitely improve the developer experience while writing scripts, regardless if the user is using typescript or not.

Originally posted by @simskij in #1 (comment)