feat: Making the default config file empty, writing checks of its values

Created by: whitebear60

• Make the default config file empty
• Write checks of the String config values
• Write checks of the List config values
• Check that the mod pack directory exists before creating a server pack; throw an exception if it doesn't exist.
• Write checks of the Boolean config values
• Write checks for Minecraft and mod loader versions

Implementing the feature described in #13 (closed)