Refactor SettingsManager to use an existing library

[AHeroicLlama]

SettingsManager class was written totally custom originally to support bespoke features such as versioning and custom warnings for malformed settings. It also mocked the .ini file format to be more familiar to technically-minded Fallout players.

However this custom setup causes overhead whenever a new setting is added to the config file.

We should consider refactoring this to use something more standard, eg system.text.json.
We must however still maintain a level of control, such that if a feature is removed but users have an "old" config file, there are no errors or confusion caused, for example we might want to control how settings are read in, should a feature change.
See fe7cb41 for previous example of a would-be breaking change being caught.