Add MemoryUtil for Apple external.

source/funkin/external/apple/project/include/MemoryUtil.hpp

@@ -0,0 +1,8 @@
+#pragma once
+
+/**
+ * Retrieves the current process's resident set size (RSS) in bytes on Apple platforms.
+ *
+ * @return The resident set size (RSS) in bytes if successful; otherwise, returns 0 on failure.
+ */
+size_t Apple_MemoryUtil_GetCurrentProcessRss();