Markdown support

[Matistjati]

This PR is not done, at the moment this serves as a TODO-list

supported for both pdf and html:

• problem name and id are automatically inserted at the top of the statement. Spec that we automatically prepend it from problem.yaml
• samples
• interactive samples
• commonmark-style images
• svg images. needs to be specced
• "common" footnotes. needs to be specced. (the implementer should be free to choose how they look and where they are placed)
• "common" tables. needs to be specced
• inline and display math with \$a+b$ and \$$a+b$$. needs to be specced

relevant to spec: i think pandoc is compliant with https://www.markdownguide.org/extended-syntax/ (specifies footnotes and tables)

TODO:

• add nextsample and remainingsamples

TODO testing:

• are the dependencies correct on all systems?
• make a "maximal" example (?)
• debug overflowing line on team coding (more testing)
• "fuzz test" (how?)
• do script tags present security vulnerability?

not planned to be added in this PR (maybe ever):

• tex -> html using pandoc (remove plastex)
• tex -> pdf (we will keep using pdflatex for this)
• pdf images