Added support for "@" in identifiers

This is to resolve the following error on a typescript file:

[main] ERROR net.sourceforge.pmd.cli - Error while tokenizing: Lexical error in file '.\TableList.js' at
 line 67, column 1: "@" (64), after : "" (in lexical state DEFAULT)

pmd-javascript/etc/grammar/Ecmascript5.jj

@@ -281,7 +281,8 @@ TOKEN :
     |
         [
           "$",
-          "_"
+          "_",
+          "@"
         ]
     |
         <UNICODE_ESCAPE_SEQUENCE>