Publish TypeScript declaration files: `.d.ts`

[piotrl]

Even though this library is written in TypeScript, when included in other packages (like Angular), typings can't be reached and it's treated as normal JavaScript library.

To help developers out there, we can write single file with public api.

See official doc: https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html

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