Not actually shipping types

[chriskrycho]

This advertises that it's shipping types for TS to consume, but the files hash in package.json only ships dist, and nothing is copying the index.d.ts file from lib. (Just having a .d.ts file in the included directory will not get it published.) There are a couple ways to solve this:

1. Since this repo increasingly uses TS itself, just add declaration: true to the compilerOptions section of tsconfig.json and move the exported type to index.d.ts. However, doing this would require a breaking change, as it would require compiling to ES Modules or otherwise switching away from the export = setup currently used in index.ts.
2. Move it to the root of the repo and include it in files.
3. Manually copy it as part of the build process.

I'm happy to open a PR that does any of these.