Add built-in types

Not sure why it seems like the 'is-not? local' is not working.

Will investigate later

test/highlight/built-ins.tig

@@ -3,11 +3,23 @@ let
   /*       ^ function.builtin */
 
   primitive exit(ret: int) /* Shadowing the prelude-included built-in */
+  /*                  ^ type.builtin */
 
   var b := exit(0)
   /*       ^ function */
 
+  type int = string /* Shadowing the built-in type */
+  /*         ^ type.builtin */
+
+  var c : int := "This is an \"int\""
+  /*      ^ type.builtin (not sure why it isn't 'type')*/
 in
+  let
+    var c : int := "This is an int"
+    /*      ^ type.builtin (not sure why it isn't 'type')*/
+  in
+  end;
+
   exit(1);
   /* <- function */