Is your request related to a problem?

The readability of the backend (core package) codebase can really benefit from some strict typing.

Describe the solution you'd like

Migrate to typescript

Requirements:

• The migration should be able to be done incrementally
• Adjustments should be made to the dev workflow such that it isn't more cumbersome than before. We may provide npm scripts for:
  • auto recompilation of typescript source files
  • auto restart of some common dev commands (e.g. markbind s -d in the docs/ folder) run after recompilation
  • can look into: https://www.npmjs.com/package/tsc-watch
  • ❔
• Other workflows should be adapted as necessary: release, automated site deployment, ...

Additional context

The frontend may not see as much benefit, since we only develop mostly independent components. Also, typescript support for vue v2 apis isn't known to be strong.

JSDoc typing: #1039 (review)