Simplify setup

[LinusU]

Hey @Gozala,

I know that this is basically a rewrite of the entire package, but I wanted to see if there is interest in merging this upstream instead of me publishing my own package ☺️

The motivation behind this is that I couldn't use the package out of the box since the module field points to a file with Flow-typings in it, and thus I cannot consume it without a flow compile step (e.g. via Babel). When I started to fix it I found that the .d.ts file contained more than just typings, it contained the entire module again, so then I wanted to fix that also. In the end, I found it easier to sort of start from scratch...

If you want to, I would be happy if you would like to merge this PR and add me as a maintainer with publishing rights to Npm.

You can also feel free to not merge this, there will be no hard feelings and I'll just publish it under another name!

[LinusU]

I ended up publishing my version as a separate package: https://github.com/LinusU/ts-unreachable