Core: Add a new File class, obtainable from an Archive, and a stub implementation.

src/core/file_sys/archive_romfs.h

@@ -28,6 +28,14 @@ public:
      */
     IdCode GetIdCode() const override { return IdCode::RomFS; };
 
+    /**
+     * Open a file specified by its path, using the specified mode
+     * @param path Path relative to the archive
+     * @param mode Mode to open the file with
+     * @return Opened file, or nullptr
+     */
+    std::unique_ptr<File> OpenFile(const std::string& path, const Mode mode) const override;
+
     /**
      * Read data from the archive
      * @param offset Offset in bytes to start reading data from