Slate markdown serializer

Motivation

At first we wanted to use Netlify's slate-markdown-serializer since it does exactly what we need.

There were, however, several tiny issues:

It does not seem to be updated anymore (last update is from two years ago, but this could have been solved using tommoor's fork)
Both libraries reuse parts marked which prevents simple updates
Both libraries dictate type string for Block, Inline and Mark
Both libraries lack TypeScript support