Regular Expression Denial of Service in markdown

[Piotr Hebda]

• I'm submitting a ...
  [ ] language request
  [ x ] bug report
  [ ] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

• Summary
  GHSA-wx77-rp39-c6vg

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)
  Please switch from unmaintained markdown package to something else. I'm not feeling in a good position to propose any alternatives.

[Fengda Huang]

Would you like to give a PR ?