Add external keyword to the tokenizer.

lib/ace/mode/xquery_highlight_rules.js

@@ -30,6 +30,7 @@ var XQueryHighlightRules = function() {
      "switch|default|try|catch|group|tumbling|sliding|window|start|end|at|only|" +
      "using|stemming|" +
      "while|" + 
+     "external|" +
      "if|then|else|as|and|or|typeswitch|case|ascending|descending|empty|in|count|updating|insert|delete|replace|value|node|attribute|text|element|into|of|with|contains").split("|")
     );