Try to move external-types

[DerMolly]

We should try to move as much as possible from src/external-types to either the projects repos or DefinetlyTyped.

• [x] abcjs PR there
• [x] emoji-mart
  • unnecessary as of #620
• [x] flowchart.js PR there | PR here
• [ ] markdown-it-abbr
• [ ] markdown-it-deflist
• [ ] markdown-it-emoji
• [ ] markdown-it-footnote
• [x] markdown-it-front-matter PR there
• [ ] markdown-it-imsize
• [ ] markdown-it-ins
• [ ] markdown-it-linkify
• [ ] markdown-it-mark
• [ ] markdown-it-mathjax PR there
• [x] markdown-it-plantuml PR there
• [ ] markdown-it-regex PR there
• [ ] markdown-it-sub PR there
• [ ] markdown-it-sup PR there
• [x] markdown-it-toc-done-right PR there | PR here
• [x] markmap-lib
  • afaik this is not possible to remove as it is needed to function correctly, but it's a reexport anyway so maybe that is okay-ish.

[mrdrogdrog]

Please be sure, that

• Every PR includes EVERY possible type, signature and option! For our project it's okay to just use what we need. But if you provide these types to the original project then add everything. Otherwise they're useless because other devs have to complete them for their projects.
• You use function instead of lambda consts, because this is something that only we do.

[DerMolly]

Most of these were attempted, but many repos did adopt this unfortunatly