Add types field to package.json

[Blasz]

This allows the types in the repo to be consumed by third parties.

[wclr]

is it supposed to?

[Blasz]

Well, the project is written in typescript and emits declaration files, so it is the recommended approach to expose these types directly for consumers, see https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html.

I'm consuming yalc via node rather than using the CLI so I'd like to leverage the types of the functions that are exported from index.