[markdown mode] Allow uppercase X in task list syntax
[markdown mode] Allow uppercase X in task list syntax

Closes #4960

jkaplon/CodeMirror,jkaplon/CodeMirror,thomasjm/CodeMirror,dperetti/CodeMirror,jayaprabhakar/CodeMirror,MarcelGerber/CodeMirror,dperetti/CodeMirror,jayaprabhakar/CodeMirror,mtaran-google/CodeMirror,pabloferz/CodeMirror,thomasjm/CodeMirror,SidPresse/CodeMirror,notepadqq/CodeMirror,wingify/CodeMirror,namin/CodeMirror,hackmdio/CodeMirror,dperetti/CodeMirror,pabloferz/CodeMirror,MarcelGerber/CodeMirror,wingify/CodeMirror,SidPresse/CodeMirror,jayaprabhakar/CodeMirror,thomasjm/CodeMirror,ficristo/CodeMirror,codio/CodeMirror,hackmdio/CodeMirror,mtaran-google/CodeMirror,namin/CodeMirror,notepadqq/CodeMirror,namin/CodeMirror,hackmdio/CodeMirror,jkaplon/CodeMirror,nightwing/CodeMirror,codio/CodeMirror,pabloferz/CodeMirror,wingify/CodeMirror,MarcelGerber/CodeMirror,ficristo/CodeMirror,notepadqq/CodeMirror,SidPresse/CodeMirror,nightwing/CodeMirror,codio/CodeMirror,ficristo/CodeMirror,mtaran-google/CodeMirror,nightwing/CodeMirror