[javascript mode] Also allow a regexp after 'default'
[javascript mode] Also allow a regexp after 'default'

(For 'export default' syntax)

Issue #4452

SidPresse/CodeMirror,hackmdio/CodeMirror,jayaprabhakar/CodeMirror,hackmdio/CodeMirror,nightwing/CodeMirror,grzegorzmazur/CodeMirror,namin/CodeMirror,codio/CodeMirror,kerabromsmu/CodeMirror,jayaprabhakar/CodeMirror,MarcelGerber/CodeMirror,xeronith/CodeMirror,ficristo/CodeMirror,SidPresse/CodeMirror,mtaran-google/CodeMirror,notepadqq/CodeMirror,crazy4chrissi/CodeMirror,schanzer/CodeMirror,thomasjm/CodeMirror,schanzer/CodeMirror,jkaplon/CodeMirror,jkaplon/CodeMirror,wingify/CodeMirror,pabloferz/CodeMirror,ficristo/CodeMirror,dperetti/CodeMirror,namin/CodeMirror,kerabromsmu/CodeMirror,nightwing/CodeMirror,crazy4chrissi/CodeMirror,kerabromsmu/CodeMirror,mtaran-google/CodeMirror,wingify/CodeMirror,codio/CodeMirror,rrandom/CodeMirror,crazy4chrissi/CodeMirror,ejgallego/CodeMirror,dperetti/CodeMirror,thomasjm/CodeMirror,ficristo/CodeMirror,schanzer/CodeMirror,ejgallego/CodeMirror,MarcelGerber/CodeMirror,pabloferz/CodeMirror,pabloferz/CodeMirror,xeronith/CodeMirror,xeronith/CodeMirror,notepadqq/CodeMirror,notepadqq/CodeMirror,SidPresse/CodeMirror,grzegorzmazur/CodeMirror,thomasjm/CodeMirror,jayaprabhakar/CodeMirror,wingify/CodeMirror,rrandom/CodeMirror,MarcelGerber/CodeMirror,nightwing/CodeMirror,hackmdio/CodeMirror,namin/CodeMirror,ejgallego/CodeMirror,dperetti/CodeMirror,mtaran-google/CodeMirror,codio/CodeMirror,rrandom/CodeMirror,grzegorzmazur/CodeMirror,jkaplon/CodeMirror