Add support for creating empty save files

[maddymakesgames]

This should be implemented by adding Default implementations for all SaveData-related structs and then adding a button in the GUI app. It should have all vanilla data loaded in by default but zeroed out and have no modded data.

The new save file should be named [n+1].celeste where n is the largest open save file (on web) or the largest save file in the saves folder (on native).