Add generated types declaration for minimal typescript support

[Shelob9]

Typescript support would be great. Automatically generating types, without having to fully re-write this in typescript is an easier first step.

To build this file -- in another projec that had wpapi installed, I added a directory called "types" than I ran npx dts-gen -m wpapi -d wpapi. I copied types/index.d.ts into wpai.

So, if this manual way is good enough for now, then automating that process would be the next step to make this useful.

Would fix #149

[iamandrewluca]

@Shelob9 check this PR https://github.com/Shelob9/node-wpapi/pull/1 it will merge in your feature/149 branch