Config improvements (tracking issue)

[stackotter]

This is a top-level tracking issue for various config improvements I plan on making.

• Add platform-specific configuration parameters to their corresponding top-level configuration types and catch catch their presence during validation so that we can throw a nice descriptive error rather than Codable's 'encountered unexpected key' error. Currently they are only present in the configuration type's overlay type rather than the top-level type itself.
• Suggest similar configuration keys when it's possible that the user has typoed a configuration key
• Introduce interactive commands for editing configuration files. This will probably require adding trivia preservation support to toml++ and may be better suited in a separate PR.
• Centralise migration-related code to simplify regular configuration-related code.
• Document configuration macros for contributors
• Document configuration format for users
• Make generic enum parser so that we don't need to use swift-parsing all over the place. By enum parsing I'm referring to parsing config syntax such as "platform(macOS)".
• Maybe include line numbers along with config errors where possible?