update(conventions): extend windows conventions to show more capability.

include/conventions/base_convention.hpp

@@ -1,5 +1,6 @@
 #pragma once
 #include <string>
+#include <algorithm>
 #include <utility>
 
 #ifndef BASE_CONVENTION_HPP
@@ -31,4 +32,18 @@ namespace argument_parser::conventions {
     using convention = base_convention; 
 }
 
+namespace argument_parser::conventions::helpers {
+    static std::string to_lower(std::string s) {
+        std::transform(s.begin(), s.end(), s.begin(),
+            [](unsigned char c) { return std::tolower(c); });
+        return s;
+    }
+
+    static std::string to_upper(std::string s) {
+        std::transform(s.begin(), s.end(), s.begin(),
+            [](unsigned char c) { return std::toupper(c); });
+        return s;
+    }
+}
+
 #endif