feat(vcs): Accept a diff change from the keyboard and step between diff files (#158)

- Accept Left Side / Accept Right Side copy the change at the caret to the
  other side of a side-by-side diff (⌃⌘→ / ⌃⌘← on macOS, Ctrl+Alt+R /
  Ctrl+Alt+A on Windows).
- Compare Next File / Compare Previous File open the neighbouring file of
  the diff chain directly (⌃⇧→ / ⌃⇧← on macOS, Alt+Shift+→ / ← on Windows).

src/features/vcs/diffNavigator.ts

@@ -4,6 +4,10 @@ export interface DiffNavigator {
   next(): void;
   prev(): void;
   jumpToSource(): void;
+  /** Apply the change at the caret from the left side to the right (`a`) or back (`b`). */
+  applyChange(source: "a" | "b"): boolean;
+  /** Open the next / previous file of the diff chain (changes of a commit or change list). */
+  file(dir: 1 | -1): void;
 }
 
 let focused: DiffNavigator | null = null;