Mod file overlay system

Mods can now replace DVD files with contents of their "overlay" folder

(I'll update the docs later when I do a full pass and make non-code mods
more of a first-class citizen)

Fixes https://github.com/TwilitRealm/dusklight/issues/1306

include/dusk/mod_loader.hpp

@@ -67,6 +67,7 @@ private:
 
     void tryLoadDusk(const std::filesystem::path& modPath, bool fromDir);
     void buildAPI(LoadedMod& mod);
+    void initOverlayFiles();
 };
 
 }  // namespace dusk