restore option as no-op to not break the world

Author: Kha

src/Lean/Language/Lean.lean

@@ -355,6 +355,11 @@ private def getNiceCommandStartPos? (stx : Syntax) : Option String.Pos.Raw := do
     stx := stx[1]
   stx.getPos?
 
+register_builtin_option experimental.module : Bool := {
+  defValue := false
+  descr := "no-op, deprecated"
+}
+
 /--
 Entry point of the Lean language processor.