Refactor source control code

[bummoblizard]

Plan:

1. Migrate to using async/await pattern as opposed to completion handlers in LocalGitServiceProvider
2. Design better abstraction and implement local Git features as an extension

This allows us to:

• Implement additional Git features without increasing complexity of the core code
• Implement Git over SSH #886

[bummoblizard]

Extension API ideas:

• [x] ActivitybarManager (Sidebar) to dynamically add sidebar sections
• [ ] FileDecoration API to add labels (git status) to file cells
• [x] StatusBar API to add labels & indicators to the bottom bar
• [ ] Other necessary callbacks to support diff features

While implementing these new features, explore the possibility of using generics in our APIs:

• https://developer.apple.com/videos/play/wwdc2022/110352/
• https://developer.apple.com/videos/play/wwdc2022/110353/
• https://developer.apple.com/videos/play/wwdc2023/10168/

[bummoblizard]

Example of using generics to abstract the common operations of extension contribution points (ToolBar, StatusBar etc.): https://github.com/thebaselab/codeapp/blob/main/CodeApp/Managers/ExtensionViewItemManager.swift

protocol CodeAppContributionPointManager: ObservableObject {
    associatedtype Item where Item: Identifiable, Item.ID == UUID
    var items: [Item] { get set }
    func registerItem(item: Item)
    func deregisterItem(id: UUID)
}

extension CodeAppContributionPointManager {
    func registerItem(item: Item) {
        items.append(item)
    }
    func deregisterItem(id: UUID) {
        items.removeAll(where: { $0.id == id })
    }
}

class StatusBarManager: CodeAppContributionPointManager {
    @Published var items: [StatusBarItem] = []
}

class ToolbarManager: CodeAppContributionPointManager {
    @Published var items: [ToolbarItem] = []
}

// .. etc