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