sh.kak: Use a region for shell expansions
sh.kak: Use a region for shell expansions

Fixes #2763

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