add ML_system_pp for type Isabelle_Thread.T;

--- a/src/Pure/ML/ml_pp.ML	Wed Sep 06 14:09:27 2023 +0200
+++ b/src/Pure/ML/ml_pp.ML	Wed Sep 06 16:03:22 2023 +0200
@@ -8,6 +8,11 @@
 struct
 
 val _ =
+  ML_system_pp (fn _ => fn _ => fn t =>
+    PolyML.PrettyString ("<thread " ^ quote (Isabelle_Thread.print t) ^
+      (if Isabelle_Thread.is_active t then "" else " (inactive)") ^ ">"));
+
+val _ =
   ML_system_pp (fn _ => fn _ => Pretty.to_polyml o Proof_Display.pp_context);
 
 val _ =