await openEditorAtIndex1 command (#2442)
await openEditorAtIndex1 command (#2442)

Previously there was a bug in VSCode which prevented me from doing:
```
await
vscode.commands.executeCommand('workbench.action.openEditorAtIndex1')
```

The most recent release of VSCode (1.21.0) fixes the bug. See
https://github.com/Microsoft/vscode/issues/44636
VSCodeVim/Vim,Chillee/Vim,Chillee/Vim,VSCodeVim/Vim