src: Allow face names to contain an underscore
src: Allow face names to contain an underscore

Closes #2229

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