Improves thread-safety and ensures that FileConfigs reload properly

[TheElectronWill]

The goal is to resolve #147 by doing multiple things:

• add thread-safe config types and make FileConfig use them
• add a throttling to FileConfig autosave (autoreload has been taken care of in #145)
• avoid reloading a FileConfig if the file is empty, wait a little bit for the editor to write the file (there might be a way to detect that the file is in use)

Hopefully this work can also provide #64 :wink:

[TheElectronWill]

Here is an example of the new capabilities of FileConfigs: https://github.com/TheElectronWill/night-config/issues/147#issuecomment-1913645866