Properly include and export TypeScript types

[nex3]

We need to copy the entrypoing typings file, as described in the
TypeScript docs:

It’s important to note that the CommonJS entrypoint and the ES
module entrypoint each needs its own declaration file, even if the
contents are the same between them. Every declaration file is
interpreted either as a CommonJS module or as an ES module, based on
its file extension and the "type" field of the package.json, and
this detected module kind must match the module kind that Node will
detect for the corresponding JavaScript file for type checking to be
correct. Attempting to use a single .d.ts file to type both an ES
module entrypoint and a CommonJS entrypoint will cause TypeScript to
think only one of those entrypoints exists, causing compiler errors
for users of the package.

Closes #247