add API review file and build step

[chrisradek]

This is just a proof of concept. I wanted an easy way to look at the public API of our library according to what types we expose. I'm using api-extractor pointing at the entry-point of our type definitions to generate the API markdown file.

Using this would help allow us to easily detect when the API surface changes, areas for improving our documentation, and detects if we've forgotten to export anything that's referenced by our public API.

Also worth going through to make sure we aren't exporting something that's not actually usable/intended to be used.