[Variadic] Export Variadic.Options through the API

src/plugins/variadic/Variadic.mli

@@ -20,4 +20,12 @@
 (*                                                                        *)
 (**************************************************************************)
 
-(* Nothing is exported statically. *)
+module Options: sig
+  (** When enabled, the plugin traverses the current AST, and translates
+      variadic functions, references to va_arg and variadic builtins to a
+      semantically code. *)
+  module Enabled : Parameter_sig.Bool
+  (** In strict mode, non-portable casts between integral types are forbidden
+      in calls to LibC's variadic functions. *)
+  module Strict : Parameter_sig.Bool
+end