Change the drracket repl so that it doesn't show the stacktrace button

for syntax errors (i.e., exn:fail:syntax? exceptions)

Overall, it seems important to (in most cases) not show the stack that
leads to the point where raise-syntax-error is called, so as not to
confuse users who might think that that stack is useful information
in understanding how to fix the syntax error.

I've resisted this technique for avoiding those stacks for a long
time, mostly because common syntax errors in the standard settings
don't usually get the stacktrace button because they usually have
empty stacks (partly thanks to the with-stack-checkpoint dance).

But now that the underlying racket-generated stack is shown too, and
not just the errortrace-generated stack, it is harder to make those go
away. More precisely, DrRacket used to be able to rely on the fact
that files that were compiled to .zo files would not have the
instrumentation in them and thus stackframes from that code would not
show up in the stacktrace. No longer.

So, it seems time to try this alternative.

collects/drracket/private/debug.rkt

@@ -30,6 +30,9 @@ profile todo:
 (define orig (current-output-port))
 (define (oprintf . args) (apply fprintf orig args))
 
+(define base-phase
+  (variable-reference->module-base-phase (#%variable-reference)))
+
 (provide debug@)
 (define-unit debug@
   (import [prefix drracket:rep: drracket:rep^]
@@ -297,9 +300,9 @@ profile todo:
 
       (print-planet-icon-to-stderr exn)
       (unless (exn:fail:user? exn)
-        (unless (and (null? stack1)
-                     (null? stack2))
-          (print-bug-to-stderr msg stack1 stack1-editions stack2 stack2-editions defs ints))
+        (unless (exn:fail:syntax? exn)
+          (unless (and (null? stack1) (null? stack2))
+            (print-bug-to-stderr msg stack1 stack1-editions stack2 stack2-editions defs ints)))
         (display-srclocs-in-error src-locs src-locs-edition))
       (display msg (current-error-port))
       (when (exn:fail:syntax? exn)