Add all class_names as manual preloads to all processed gdscripts

[dugramen]

Classes can't be added to the global namespace if they are not in the same directory. Instead this can be worked around and emulated by doing

const MyClass := preload("path_to_my_class.gd")

So when processing scripts at compile/save time, gather all global classes, and add them to the start of every processed script (after @tool and class_name)