[javascript mode] Recognize typescript declare keyword

Closes #4915

Author: marijnh

mode/javascript/javascript.js

@@ -363,6 +363,9 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       if (isTS && value == "type") {
         cx.marked = "keyword"
         return cont(typeexpr, expect("operator"), typeexpr, expect(";"));
+      } if (isTS && value == "declare") {
+        cx.marked = "keyword"
+        return cont(statement)
       } else {
         return cont(pushlex("stat"), maybelabel);
       }