Convert to typescript (prepare to upgrade all dependencies)

[thdk]

Hi!

We've noticed the library is a bit outdated and we would like to upgrade the dependencies. Would you be up for using typescript and jest (instead of tap)?

Using typescript gives us more comfort for doing the upgrades as the types from the dependencies might already indicate where we should pay extra attention.

This PR should not contain any code changes or dependency upgrades yet. It's just adding the minimum type info. Add tooling for building and testing typescript files. And run prettier.

I've also included a github action to run CI.

Feel free to let me know in which changes you would be interested so I can leave the other changes out.