Create a playground

[pmeinhardt]

I quickly threw together an interactive ADF-to-Markdown playground as outlined in #17. The aim of this is to provide a way to try out the package functionality on user input. Apart from demo purposes, this might also come in handy during development.

This is still a very rough work-in-progress (hence the draft PR status). ✏️

But please give it a try.

Do note that the playground does not automatically rebuild when the plugin code is updated.

To do:

• Update ESLint configuration: playground files, i.e. .tsx files are not inspected at the moment 🔎
• Maybe this could be deployed somewhere or made available via sth. like CodePen or similar so interested people can give it a try/see how it mightbe useful for them 🤔

Closes #17.