Consider exposing TaxJar Types definitions.

[caioquinta]

I'm using Typescript (4.0.5) and since I don't want to import them directly from the dist directory like proposed in this issue.
I really don't feel comfortable with this approach since the dist folder is typically used for the transpiled files and importing from there could lead to some reliability issues.

It would be nice to have those Types definitions exposed for us to use when importing the package.

Thanks!