Fix moving backwards across astral chars
Fix moving backwards across astral chars

Closes #6544

pabloferz/CodeMirror,dperetti/CodeMirror,wingify/CodeMirror,dperetti/CodeMirror,hackmdio/CodeMirror,wingify/CodeMirror,pabloferz/CodeMirror,hackmdio/CodeMirror,dperetti/CodeMirror,hackmdio/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror,pabloferz/CodeMirror,nightwing/CodeMirror,nightwing/CodeMirror