Proposal: Improve Theme System usability

[NoahKamara]

Current problem:

the use of subscript(dynamicMember member: String) obfuscates which properties are available.
this makes it hard to get started with theming.

SwiftUI's Environment provides a nice solution to that exact problem. The result is a type safe key that is easy to implement in custom nodes and easy to use when theming.

I think it would be relatively easy adapting it to this library (I was able to do naive refactor in just a few minutes)

protocol ThemeKey {
  static var defaultValue: AttributedDict { get }
}

struct Theme {
  private var attributes: [ObjectIdentifier: AttributeDict] = [:]

    public subscript<Key: ThemeKey>(_ key: Key.Type) -> AttributeDict {
        get {
            return attributes[ObjectIdentifier(key)] ?? Key.defaultValue
        }
        set {
            attributes[ObjectIdentifier(key)] = newValue
        }
    }
}

An implementation for the link node could look like this:

fileprivate struct LinkThemeKey {
  static let defaultValue: AttributedDict = [.foregroundColor: UIColor.blue]
}

extension Theme {
  var link: AttributedDict {
    get { 
      self[LinkThemeKey.self] 
    }
    set { 
      self[LinkThemeKey.self] = newValue
    }
  }
}

Is there an interest in moving the library towards a more typed approach like this?
I would be willing to work on this myself