Fork markdown-it-toc-done-right

[DerMolly]

While working on https://github.com/hedgedoc/react-client/issues/815, we noticed that the package markdown-it-toc-done-right needed an update to work for us. This was implemented, but never merged. I guess we don't have a choice, but to fork this into hedgedoc/markdown-it-plugins.

[DerMolly]

Part of this could be hedgedoc/hedgedoc#2946

[mrdrogdrog]

Reason of forking: nagaozen/markdown-it-toc-done-right#66