Address review: add to_underlying function for strong typedefs

Registers a to_underlying() function for each strong typedef that
returns the wrapped base value from the Dynamic_Object's __value attr.

Requested by @lefticus in PR #680 review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/language/chaiscript_eval.hpp

@@ -922,6 +922,20 @@ namespace chaiscript {
           throw exception::eval_error("Type alias redefined '" + e.name() + "'");
         }
 
+        dispatch::Param_Types to_underlying_param_types(std::vector<std::pair<std::string, Type_Info>>{
+            {new_type_name, user_type<dispatch::Dynamic_Object>()}});
+
+        auto to_underlying_body = dispatch::make_dynamic_proxy_function(
+            [](const Function_Params &t_params) -> Boxed_Value {
+              const auto *obj = static_cast<const dispatch::Dynamic_Object *>(t_params[0].get_const_ptr());
+              return obj->get_attr("__value");
+            },
+            1,
+            std::shared_ptr<AST_Node>(),
+            to_underlying_param_types);
+
+        t_ss->add(to_underlying_body, "to_underlying");
+
         return void_var();
       }
     };

unittests/strong_typedef.chai

@@ -43,3 +43,13 @@ try {
 } catch(e) {
   // Expected: Seconds is not Meters
 }
+
+// to_underlying should return the base value
+assert_equal(to_underlying(m), 42)
+assert_equal(to_underlying(s), 10)
+
+// to_underlying result should be a plain value, not a strong typedef
+def takes_int(int i) {
+  return i
+}
+assert_equal(takes_int(to_underlying(m)), 42)