File browser should be replaced by Lua implementation + tech debt cleanup

[sfan5]

We should replace the Irrlicht file browser with a Lua implementation (or perhaps with SDL_ShowOpenFileDialog since SDL3?).

Steps:

1. Replace Irrlicht file picker with new Lua dialog #13512 needs to be adopted
2. src/gui/guiPathSelectMenu.cpp and irr/src/CGUIFileOpenDialog.cpp can be deleted entirely
3. multibyteToWString in irrString.h can be deleted too
4. perhaps even some parts of CFileSystem can go

[appgurueu]

If possible I'd very much prefer a native file picker. Should offer better UX (especially on platforms like Android) and less work.