Add TypeScript definitions

[alexgavrusev]

Hi, taking a look at how to make the Capsize integration with Chakra even better, I thought it'd be a good idea to include types in this package.

Initially thought about doing this through declaration files, but Preconstruct failed to bundle them. Trying to set up a @types package also didn't work — they do not allow esModuleInterop: true that is assumed by Chakra

So I've rewritten the index and theme exports to be in TypeScript, while moving all core logic to utils.js without any further modification.

[focux]

This is awesome man! I also need typescript definitions, is possible for you to review this PR? @ceteio

[VinSpee]

It's been a while, could we get this merged if you have a moment @ceteio?