Mention the subtype of QAST::Regex during --target=ast.

Author: pmurias

src/QAST/Regex.nqp

@@ -25,7 +25,7 @@ class QAST::Regex is QAST::Node {
     method max(*@value)       { $!max := @value[0] if @value; $!max }
 
     method dump_extra_node_info() {
-        ":rxtype($!rxtype)"
+        ":rxtype($!rxtype)" ~ (!nqp::isnull_s($!subtype) ?? " :subtype($!subtype)" !! "")
     }
 }