[javascript mode] Recognize typescript declare keyword
[javascript mode] Recognize typescript declare keyword

Closes #4915

codio/CodeMirror,mtaran-google/CodeMirror,wingify/CodeMirror,thomasjm/CodeMirror,SidPresse/CodeMirror,notepadqq/CodeMirror,SidPresse/CodeMirror,codio/CodeMirror,pabloferz/CodeMirror,SidPresse/CodeMirror,namin/CodeMirror,namin/CodeMirror,codio/CodeMirror,pabloferz/CodeMirror,wingify/CodeMirror,dperetti/CodeMirror,jkaplon/CodeMirror,dperetti/CodeMirror,wingify/CodeMirror,thomasjm/CodeMirror,mtaran-google/CodeMirror,ficristo/CodeMirror,pabloferz/CodeMirror,ficristo/CodeMirror,nightwing/CodeMirror,dperetti/CodeMirror,jayaprabhakar/CodeMirror,notepadqq/CodeMirror,jayaprabhakar/CodeMirror,hackmdio/CodeMirror,mtaran-google/CodeMirror,namin/CodeMirror,MarcelGerber/CodeMirror,MarcelGerber/CodeMirror,notepadqq/CodeMirror,hackmdio/CodeMirror,hackmdio/CodeMirror,thomasjm/CodeMirror,jkaplon/CodeMirror,nightwing/CodeMirror,jayaprabhakar/CodeMirror,nightwing/CodeMirror,ficristo/CodeMirror,MarcelGerber/CodeMirror,jkaplon/CodeMirror