Center cursor vertically when appropriate

When an action causes the cursor to move outside of the viewport, center
it vertically in order to better mimic Vim's behavior.

Fixes VSCodeVim#2960

src/mode/modeHandler.ts

@@ -680,6 +680,12 @@ export class ModeHandler implements vscode.Disposable {
 
     vimState.allCursors = resultingList;
 
+    // If the cursor moved out of the viewport, center it vertically
+    vimState.editor.revealRange(
+      new vscode.Range(vimState.cursorPosition, vimState.cursorPosition),
+      vscode.TextEditorRevealType.InCenterIfOutsideViewport
+    );
+
     return vimState;
   }