syntax: javascript: colorize also special values 'null' and 'undefined'

This addresses one half of https://savannah.gnu.org/patch/?9865.
Original-patch-by: Ryan Westlund <rlwestlund@gmail.com>

syntax/javascript.nanorc

@@ -13,6 +13,8 @@ color magenta "\<(break|continue|return|yield)\>"
 
 # Octal/decimal and hexadecimal numbers.
 color cyan "\<[0-9]+\>" "\<0x[0-9A-Fa-f]+\>"
+# Special values.
+color cyan "\<(null|undefined)\>"
 
 # Strings.
 color brightmagenta ""(\\.|[^"])*"" "'(\\.|[^'])*'" "`(\\.|[^`])*`"