am: Implement UserChannel parameters

Used by the Super Mairo 3D All-Stars collection.

src/core/core.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include <cstddef>
+#include <deque>
 #include <functional>
 #include <memory>
 #include <mutex>
@@ -459,6 +460,12 @@ public:
      */
     void ExecuteProgram(std::size_t program_index);
 
+    /**
+     * Gets a reference to the user channel stack.
+     * It is used to transfer data between programs.
+     */
+    [[nodiscard]] std::deque<std::vector<u8>>& GetUserChannel();
+
     /// Type used for the frontend to designate a callback for System to exit the application.
     using ExitCallback = std::function<void()>;