Add TypeScript definitions

Add TypeScript definitions for people consuming this in a TypeScript setting - e.g. a webpack.config.ts, or a vite.config.ts (as is my case).

Requires us to explicitly define the files in package.json as a result, as otherwise the index.d.ts file would not be picked up.

I've ensured that both the bin dir and the index.js still get shipped by double checking with npm pack.