Badge: Access to underlying type

Problem:
- Access to the underlying type is not provided. This limits
  metaprogramming and usage in function templates.

Solution:
- Provide public access to the underlying type.
- Add test to ensure the underlying type is accessible.

AK/Badge.h

@@ -30,6 +30,10 @@ namespace AK {
 
 template<typename T>
 class Badge {
+public:
+    using Type = T;
+
+private:
     friend T;
     constexpr Badge() = default;