base/markdown: Correct italic/bold detection regex
base/markdown: Correct italic/bold detection regex

As specified at https://daringfireball.net/projects/markdown/syntax#em
italics are made with either single asterisks/underscores, and bold is
double asterisks/underscores. Before this, single asterisks were
understood as bold, and only underscores were understood as italics;
both of which behaviors are incorrect.

casimir/kakoune,Somasis/kakoune,jjthrash/kakoune,Somasis/kakoune,lenormf/kakoune,ekie/kakoune,mawww/kakoune,jjthrash/kakoune,lenormf/kakoune,danr/kakoune,ekie/kakoune,danr/kakoune,flavius/kakoune,alexherbo2/kakoune,flavius/kakoune,Somasis/kakoune,Somasis/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,mawww/kakoune,occivink/kakoune,jkonecny12/kakoune,flavius/kakoune,jjthrash/kakoune,casimir/kakoune,ekie/kakoune,lenormf/kakoune,occivink/kakoune,jkonecny12/kakoune,occivink/kakoune,mawww/kakoune,jjthrash/kakoune,alexherbo2/kakoune,occivink/kakoune,ekie/kakoune,danr/kakoune,mawww/kakoune,flavius/kakoune,lenormf/kakoune,alexherbo2/kakoune,danr/kakoune,casimir/kakoune,alexherbo2/kakoune