Improve ergonomics of defining setting keys

[lukaskubanek]

Current Approach

Keys use the SettingKey<Value> class, which inherits from SettingKeys. This is to allow grouping key definitions under the same namespace as follows:

extension SettingKeys {
  static let appLaunchCount = SettingKey<Int>(
    "MyAppLaunchCount",
    defaultValue: 42
  )
}

This structure enables autocomplete and semantic grouping. The tradeoff is extra boilerplate and the need to use classes and inheritance for what are conceptually just values.

Alternatives Considered

• Constrained extensions on SettingKey – Struct-based and simple, but lacks grouping of related keys. Used by Point-Free’s Sharing library.
• Dynamic member lookup – Only useful for getter/setter. Doesn’t support reset, observation, or publisher APIs.
• Key paths from SettingKeys to SettingKey – Too verbose and indirect. Cannot use SettingKey directly.
• Protocol-based SettingKeys – Protocols can't hold stored properties. Would need to go with computed properties.
• Macros – Seems too heavy-weight for this use case.