fix: add superagent types as prod dependency

[43081j]

We expose superagent's types in our own types, so this should be a production dependency. Otherwise, we will cause errors in the typescript builds of consumers (unless they skip lib check).

It does mean we add an unnecessary dependency for non-ts people, but there's not any simple way around that.

Fixes #346

[jthemphill]

Thank you!

[jthemphill]

unless they skip lib check

I think we do skip lib check... the issue is that our own project doesn't typecheck because we're relying on types that we couldn't find in our dependency tree.