Create Relm4 components for document-based apps

[chriskilding]

SwiftUI has building blocks specifically for making document-based apps (like text editors). These remove a huge amount of boilerplate code for opening/saving of files, undo/redo, and so on.

In SwiftUI, these building blocks allow you to create a document-based app with as little code as:

@main
struct DocumentBasedApp: App {
    var body: some Scene {
        DocumentGroup(newDocument: { ChecklistDocument() }) { configuration in
            ChecklistView()
        }
    }
}

(More documentation: https://developer.apple.com/documentation/swiftui/building_a_document-based_app_with_swiftui)

It would therefore be great to see some equivalent building blocks for creating document-based apps in Relm4.