✨ Code Review Ribbon

A code review ribbon needs to exist in the ruler view that both indicates change and provides actions the user can take on these changes including:
- Show Change - which will show the user the original code before the change above
- Discarding Change - which will remove the changed code and replace it with what existed before the change

See below screenshots to see how this might look (or refer to Xcode for reference)

Note: This may require #127 to be complete in order to begin working on this.

先查看 issue #127，然後使用連結的螢幕截圖和 Xcode 參考來了解預期的 ruler-view 互動。完成的標準是 ruler 顯示程式碼變更，並提供 Show Change 和 Discard Change 動作，以顯示或還原原始程式碼。

由索引模型根據 Issue 內容生成。