[markdown mode] Add HTML comment support
[markdown mode] Add HTML comment support

HTML comments are generally permissible in Markdown, per https://spec.commonmark.org/0.28/.
mtaran-google/CodeMirror,mtaran-google/CodeMirror,SidPresse/CodeMirror,dperetti/CodeMirror,notepadqq/CodeMirror,namin/CodeMirror,pabloferz/CodeMirror,codio/CodeMirror,codio/CodeMirror,pabloferz/CodeMirror,hackmdio/CodeMirror,jkaplon/CodeMirror,namin/CodeMirror,dperetti/CodeMirror,notepadqq/CodeMirror,jkaplon/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror,nightwing/CodeMirror,SidPresse/CodeMirror,SidPresse/CodeMirror,namin/CodeMirror,pabloferz/CodeMirror,hackmdio/CodeMirror,wingify/CodeMirror,codio/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror,hackmdio/CodeMirror,mtaran-google/CodeMirror,thomasjm/CodeMirror,jkaplon/CodeMirror,thomasjm/CodeMirror,thomasjm/CodeMirror,dperetti/CodeMirror,notepadqq/CodeMirror