Experiment with adding Typescript type declarations

[jimpick]

We want to build the library with vanilla javascript so it runs in web browsers without a compile step, but many applications will want to use TypeScript for stronger types. We can add a declaration file:

https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html

The @filecoin-shipyard/js-lotus-client-rpc library dynamically handles the method invocations... one approach might be to do some code generation from the schema in order to generate the declarations.