Add tsd(?)

[anthonyjoeseph]

'tsd' is a utility for writing tests for type definitions

It's like dtslint (which is what fp-ts uses) minus the outdated linter

I first ran across it in ts-adt

I'm not sure if it's common for fp-ts libraries to have complicated enough types that it's worth writing a separate suite of tests for them, but it seems like it might be worth looking into at least