fixes nimPreviewSlimSystem; register echoBinSafe for nimPreviewSlimSystem (#20194)

* register echoBinSafe

* add output

lib/system.nim

@@ -3147,6 +3147,8 @@ when not defined(nimPreviewSlimSystem):
   {.deprecated: "io is about to move out of system; use `-d:nimPreviewSlimSystem` and import `std/syncio`".}
   import std/syncio
   export syncio
+else:
+  import std/syncio
 
 when not defined(createNimHcr) and not defined(nimscript):
   include nimhcr

tests/system/tslimsystem.nim

@@ -0,0 +1,6 @@
+discard """
+  output: "123"
+  matrix: "-d:nimPreviewSlimSystem"
+"""
+
+echo 123