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.

[the TypeScript docs]: https://www.typescriptlang.org/docs/handbook/esm-node.html

Closes #247