Provide API & typings

[bajtos]

Allow Station Core embedders (e.g. Station Desktop) to import type declarations of data structures produced by Station Core (e.g. ActivityEvent).

Make sure the main and typings files described in package.json actually exist.

https://github.com/filecoin-station/core/blob/cdebdbbc3c163386097369fb1ec989eadd90b787/package.json#L10-L11

[juliangruber]

These files used to exist, it's a regression